As Anindo says, USB is a complex protocol which requires a microcontroller or specific USB IC, such as the FT245, FT232 USB to parallel/serial chips. If you want to and are willing to learn, then grab a starter USB dev board from e.g. Microchip or Atmel (e.g. PIC18F, 24F or PIC32 based) a couple of basic electronics books and learn the basics (plenty of questions on here about getting started with this stuff, have a look around)
If you are not looking to learn about circuit design or microcontrollers, which from scratch would take a considerable amount of time and effort to accquire the skills you would need for Home Automation type stuff, then my advice is to purchase something like a USB I/O or logging type device or Multimeter with PC comms capability.
National Instruments do a range of USB data logging products which can deal with digital or analogue inputs, plus outputs for controlling relays, etc. Here is an example of one; a digital I/O with up to 96 lines. Another is the Compact DAQ system for sensor logging, which has various sensor modules that can be used with it. Have a browse around the main data acquisition pages to see if anything suits your needs.
For simpler and cheaper more basic devices, see the Anatrek options.
Whatever you choose, it's quite likely you will still have to build some basic circuit to interface with the logging device. We would need to know the details of exactly what you want to do to provide a specific solution, such as whether you want to use wiring or RF, whether you want to measure voltage/currents and of what levels and frequency, etc. To figure out if something is switched on or off is easy enough, if that's all you want to do then tell us what it/they is/are, and whether you want to do it wirelessly or not (plus any other details you can think of which may help)