One of the major strength and a reason for Raspberry Pi success has been the General Purpose Input/Output (GPIO) access offered by the mini computer, these programmable in/out pins allows a great of hackability letting users expand the system and get it interacting with other hardware devices. The GPIO pin's are confusing as they are not numbered serially, here are two nice RasPi GPIO pinout layouts for making things easier to understand.

Raspberry Pi GPIO Pin Layout (Revision 1 Board)