Ethical hacking is where a person hacks to find weaknesses in
a system and then usually patches them. For example, a bank may pay a hacker to
hack their systems to see if it is hackable. If he gets in, then they know
there is potential for other people to hack in, and usually they will work with
this ethical hacker to patch these holes. If he doesn't get in, then they pray
that nobody is better at hacking than him.
Let me add this.
Hacking is simply exploring a computer's designed features, and learning how to
exploit or take advantage of those features. As an example. a computer is
designed to load instructions into memory using clearly defined rules, outlined
in an RFC that spells out how to load something into memory.