###Not recommended officially Arduino (the company) does not recommend directly supplying 5 volts in, because:

1. The target audience doesn't always understand how the schematic is designed, and as beginners/non-techs, would likey cause something bad to happen, like connecting an unregulated 5 volts to the 5V line, and blowing things, causing customer service calls/refunds/repairs/etcetera.
2. Directly supplying 5 volts bypasses the auto-sensing/voltage protection method.

###How Arduino's USB/external power selection works Supplying 5 volts directly is easy. USB power practically does this, as does the ICSP protocol/header. USB has a 500mA PTC fuse on the line, and has a p-channel mosfet, which by themselves do not offer any protection. But there is also the LMV358 labeled U5A, above that mosfet. It is (half) an opamp, being used as a comparator. If VIN is detected, and higher than 3.3 volts, the opamp drives the line low, disabling the mosfet, cutting the USBVCC off from the 5V line. This makes it so that you can use VIN and USBVCC at the same time without problems. Otherwise you would have two power sources competing on the same rail (USB and the 5 volt regulator).

###The mosfet has a body diode It's part of the mosfet construction, internal, and functions as reverse voltage protection, preventing the 5V power rail from back flowing to the USBVCC. It is disabled when the mosfet is on, and reversed biased when off.

#Warnings

1. DO NOT CONNECT USB AND YOUR 5V AT THE SAME TIME!
   By inputting a regulated 5 volts in at the 5V pin, you skip the helpful power source selection mechanism. You can just as easily connect your 5 volt in to the USB connector, or between the USB connector and the USB PTC fuse, but that will cause you to have a 500mA limit. If you need more current, you can bypass the fuse, but not the mosfet.
2. DO NOT SHORT 5V to VIN!
   The 5 volt regulator in any case, will be just dandy, as long as VIN is not used.

Not recommended officially

Arduino (the company) does not recommend directly supplying 5 volts in, because:

1. The target audience doesn't always understand how the schematic is designed, and as beginners/non-techs, would likey cause something bad to happen, like connecting an unregulated 5 volts to the 5V line, and blowing things, causing customer service calls/refunds/repairs/etcetera.
2. Directly supplying 5 volts bypasses the auto-sensing/voltage protection method.

How Arduino's USB/external power selection works

Supplying 5 volts directly is easy. USB power practically does this, as does the ICSP protocol/header. USB has a 500mA PTC fuse on the line, and has a p-channel mosfet, which by themselves do not offer any protection. But there is also the LMV358 labeled U5A, above that mosfet. It is (half) an opamp, being used as a comparator. If VIN is detected, and higher than 3.3 volts, the opamp drives the line low, disabling the mosfet, cutting the USBVCC off from the 5V line. This makes it so that you can use VIN and USBVCC at the same time without problems. Otherwise you would have two power sources competing on the same rail (USB and the 5 volt regulator).

The mosfet has a body diode

It's part of the mosfet construction, internal, and functions as reverse voltage protection, preventing the 5V power rail from back flowing to the USBVCC. It is disabled when the mosfet is on, and reversed biased when off.

Warnings

1. DO NOT CONNECT USB AND YOUR 5V AT THE SAME TIME!
   By inputting a regulated 5 volts in at the 5V pin, you skip the helpful power source selection mechanism. You can just as easily connect your 5 volt in to the USB connector, or between the USB connector and the USB PTC fuse, but that will cause you to have a 500mA limit. If you need more current, you can bypass the fuse, but not the mosfet.
2. DO NOT SHORT 5V to VIN!
   The 5 volt regulator in any case, will be just dandy, as long as VIN is not used.

###Not recommended officially Arduino (the company) does not recommend directly supplying 5v in, because:

1. The target audience doesn't always understand how the schematic is designed, and as beginners/non-techs, would likey cause something bad to happen, like connecting an unregulated 5v to the 5v line, and blowing things, causing customer service calls/refunds/repairs/etcetera.
2. Directly supplying 5v bypasses the auto-sensing/voltage protection method.

###How Arduino's USB/external power selection works Supplying 5v directly is easy. USB power practically does this, as does the ICSP protocol/header. USB has a 500mA PTC fuse on the line, and has a P-Channel Mosfet, which by themselves do not offer any protection. But there is also the LMV358 Labeled U5A, above that mosfet. It is (half) an opamp, being used as a comparator. If V_IN is detected, and higher than 3.3v, the opamp drives the line low, disabling the Mosfet, cutting the USB VCC off from the 5V line. This makes it so you can use V_IN and V_USB at the same time without problems. Otherwise you would have two power sources competing on the same rail (USB and the 5v Regulator).

###The Mosfet has a Body Diode It's part of the Mosfet construction, internal, and functions as reverse voltage protection, preventing the 5V power rail from back flowing to the USB VCC. It is disabled when the mosfet is on, and reversed biased when off.

#Warnings

1. DO NOT CONNECT USB AND YOUR 5V AT THE SAME TIME!
   By inputting a regulated 5v in at the 5V pin, you skip the helpful power source selection mechanism. You can just as easily connect your 5v in to the USB connector, or between the USB connector and the USB PTC fuse, but that will cause you to have a 500mA limit. If you need more current, you can bypass the fuse, but not the mosfet.
2. DO NOT SHORT 5V to V_IN!
   The 5V regulator in any case, will be just dandy, as long as V_IN is not used.

###Not recommended officially Arduino (the company) does not recommend directly supplying 5 volts in, because:

1. The target audience doesn't always understand how the schematic is designed, and as beginners/non-techs, would likey cause something bad to happen, like connecting an unregulated 5 volts to the 5V line, and blowing things, causing customer service calls/refunds/repairs/etcetera.
2. Directly supplying 5 volts bypasses the auto-sensing/voltage protection method.

###How Arduino's USB/external power selection works Supplying 5 volts directly is easy. USB power practically does this, as does the ICSP protocol/header. USB has a 500mA PTC fuse on the line, and has a p-channel mosfet, which by themselves do not offer any protection. But there is also the LMV358 labeled U5A, above that mosfet. It is (half) an opamp, being used as a comparator. If VIN is detected, and higher than 3.3 volts, the opamp drives the line low, disabling the mosfet, cutting the USBVCC off from the 5V line. This makes it so that you can use VIN and USBVCC at the same time without problems. Otherwise you would have two power sources competing on the same rail (USB and the 5 volt regulator).

###The mosfet has a body diode It's part of the mosfet construction, internal, and functions as reverse voltage protection, preventing the 5V power rail from back flowing to the USBVCC. It is disabled when the mosfet is on, and reversed biased when off.

#Warnings

1. DO NOT CONNECT USB AND YOUR 5V AT THE SAME TIME!
   By inputting a regulated 5 volts in at the 5V pin, you skip the helpful power source selection mechanism. You can just as easily connect your 5 volt in to the USB connector, or between the USB connector and the USB PTC fuse, but that will cause you to have a 500mA limit. If you need more current, you can bypass the fuse, but not the mosfet.
2. DO NOT SHORT 5V to VIN!
   The 5 volt regulator in any case, will be just dandy, as long as VIN is not used.

###Not recommended officially Arduino (the company) does not recommend directly supplying 5v in, because:

1. The target audience doesn't always understand how the schematic is designed, and as beginners/non-techs, would likey cause something bad to happen, like connecting an unregulated 5v to the 5v line, and blowing things, causing customer service calls/refunds/repairs/etcetera.
2. Directly supplying 5v bypasses the auto-sensing/voltage protection method.

###How Arduino's USB/external power selection works Supplying 5v directly is easy. USB power practically does this, as does the ICSP protocol/header. USB has a 500mA PTC fuse on the line, and has a P-Channel Mosfet, which by themselves do not offer any protection. But there is also the LMV358 Labeled U5A, above that mosfet. It is (half) an opamp, being used as a comparator. If V_IN is detected, and higher than 3.3v, the opamp drives the line low, disabling the Mosfet, cutting the USB VCC off from the 5V line. This makes it so you can use V_IN and V_USB at the same time without problems. Otherwise you would have two power sources competing on the same rail (USB and the 5v Regulator).

###Diode on the mosfet, is a Body Diode It's part of the Mosfet construction, internal, and functions as reverse voltage protection. It is disabled when the mosfet is on, and reversed biased when off.

#Warnings

1. DO NOT CONNECT USB AND YOUR 5V AT THE SAME TIME!
   By inputting a regulated 5v in at the 5V pin, you skip the helpful power source selection mechanism. You can just as easily connect your 5v in to the USB connector, or between the USB connector and the USB PTC fuse, but that will cause you to have a 500mA limit. If you need more current, you can bypass the fuse, but not the mosfet.
2. DO NOT SHORT 5V to V_IN!
   The 5V regulator in any case, will be just dandy, as long as V_IN is not used.

###Not recommended officially Arduino (the company) does not recommend directly supplying 5v in, because:

1. The target audience doesn't always understand how the schematic is designed, and as beginners/non-techs, would likey cause something bad to happen, like connecting an unregulated 5v to the 5v line, and blowing things, causing customer service calls/refunds/repairs/etcetera.
2. Directly supplying 5v bypasses the auto-sensing/voltage protection method.

###How Arduino's USB/external power selection works Supplying 5v directly is easy. USB power practically does this, as does the ICSP protocol/header. USB has a 500mA PTC fuse on the line, and has a P-Channel Mosfet, which by themselves do not offer any protection. But there is also the LMV358 Labeled U5A, above that mosfet. It is (half) an opamp, being used as a comparator. If V_IN is detected, and higher than 3.3v, the opamp drives the line low, disabling the Mosfet, cutting the USB VCC off from the 5V line. This makes it so you can use V_IN and V_USB at the same time without problems. Otherwise you would have two power sources competing on the same rail (USB and the 5v Regulator).

###The Mosfet has a Body Diode It's part of the Mosfet construction, internal, and functions as reverse voltage protection, preventing the 5V power rail from back flowing to the USB VCC. It is disabled when the mosfet is on, and reversed biased when off.

#Warnings

1. DO NOT CONNECT USB AND YOUR 5V AT THE SAME TIME!
   By inputting a regulated 5v in at the 5V pin, you skip the helpful power source selection mechanism. You can just as easily connect your 5v in to the USB connector, or between the USB connector and the USB PTC fuse, but that will cause you to have a 500mA limit. If you need more current, you can bypass the fuse, but not the mosfet.
2. DO NOT SHORT 5V to V_IN!
   The 5V regulator in any case, will be just dandy, as long as V_IN is not used.