Hacker are people who try to expand the boundaries of a system, improve it or otherwise make it do cool things for which this system was not designed. The people who invented MAT files, EMC and all the other neat Poser hacks are Poser hackers.

Cracker are a different beast altogether. Some people are really crackers but try to camouflage as hackers. The whole hacker/cracker mishmash is the fault of Hollywood, the distinction is way to sophisticated for studios who are only interesed in flippant stories.