What I learned at Uni today

Hackers
Access computer system or network without authorization
Breaks the law; can go to prison
Crackers
Break into systems to steal or destroy data
U.S. Department of Justice calls both hackers
Ethical hacker
Performs most of the same activities but with owner’s permission

It is hard to write a simple definition of something as varied as hacking, but I think what these activities have in common is playfulness, cleverness, and exploration. Thus, hacking means exploring the limits of what is possible, in a spirit of playful cleverness. Activities that display playful cleverness have "hack value".