Why can play my game on playstation3 from playstation 2 what do i need

The only PS3's that have backwards compatiblity were the first 60GB models, and some 80GB's that are no longer in production. You can still find them on Ebay and Amazon.

The 60GB actually has the ps2 hardware in it allowing for 98% BC, however the 80GB that is no longer in production used software to emulate ps2 playback.

This has cuase alot of people to be dissapointed with sony, however, in 2 or so years most people won't want to play ps2 games anyways, so it really doesn't matter.

Everything you would want to know is found here:
http://en.wikipedia.org/wiki/Playstation_3