The inside of a 1999 "passive" stlye keyfob. The battery is in the top-half of the fob. It makes contact with the circuit board on the two copper contact pads on the right side of the board in this picture.
This little thingy (the motion detector switch I am told...) can become dislodged from the circuit board. You can easily dislodge this if you are not careful when installing a new battery.
The exterior of a 2002 "non-passive" keyfob. This unit is from a 2002 Z06. Notice it is slimmer in profile than a "passive" fob and it does not sport the passive slide switch. Also note the "Lock" button is not round, as on the older style fob.