What do different kinds of Graphics Mean?

Practically speaking what is the difference between the graphics chip on a MacBook and MacBook Pro?

I've read about it making a difference if you're gaming, but what about for ordianary taks, does it make any difference? What if you use an additional display?

Also, are there requirements if you want to run paralell screens, do you need a certain kind of graphics?

Finally, why does the MBP have graphisc RAM and the MB doesn't?