The 100 ohm is A load - an example - what load you really use is up to you.
If you are driving a CMOS IC you do not need anything else connected to the OUT pin.
The 555 has a "push pull" or bipolar output stage. This means that it drives "actively" both high and low, so you get an output voltage whether there is a load connected or not.
The 555 load is generally connected to the OUT pin when you want a square wave output. There are other places you can connect to if you are doing less usual things.
LM555 / NE555 / xx555 datasheet - Fairchild
LM555 / NE555 / xx555 datasheet - NatSemi
Active low and high drive can be seen here:
More normal block diagram view. Here you cannot tell that the output pin is actively driven high and low.