XBOX 360 versions of games have achievements that you unlock when you accomplish certain things. Some are for missions, other are for stunt jumps, and cool things like doing a really long wheelie. Some of those achievements become impossible to get if you cheat and then save the game. But you guys don't have to worry about that because the PS3 doesn't have achievements. If you want to see what achievements are like go here - Link
They're just a fun way to make a game last longer, some people (me included) try to get as many as they can. Microsoft keeps track of the achievements that anyone connected to LIVE has gotten (even free Silver members).
The PS3 version has in-game achievements, you just don't get Xbox points for it.
It also has PS3-specific medals which are also blocked by cheating.