Raspberry Pi Board Revisions
Currently, there have been two board revisions of the Raspberry Pi, with the GPIO signals going to different pins on P1. Only three signals are different between the two boards, but it's something you need to be aware of. The GPIO signals on pins P1-03, P1-05, and P1-13 are different on the two board revisions.
Notice the signals on P1 for a revision 2 board. It shows a 5V and 3V3 power connection, along with a GND or ground connection. The other pins are labeled with the names of the GPIO signals they are attached to.
The same thing is present on a revision 1 board. Notice that some of the pins, for example pin P1-04, are not labeled. These pins are designated as Do Not Connect because the Raspberry Pi Foundation says that on future revisions they might carry different signals.
These are connected to either a power rail or ground. This has been consistent between the two revisions, so now there is talk of them being permanently assigned to these levels.
The sharp-eyed amongst you will have spotted a small difference between the two board revisions on the left hand side of the board. The revision 2 board has eight holes in the board: one with a square outline (pin 1) and the rest with round outlines. This is a place for an eight-way plug, P6, which allows you to connect to four extra GPIO pins.