Like going to a persons account and deleteing everything and like making people sad that they got hacked. Its an ok thing to do unless its not WAY TO BAD. If you play roblox search jacklee106 and play his game.
putting a virus on someones pc, smart phone, putting a RAT (remote administration tool) onto them as well, taking down a website using sql injection and over flowing it with data.
Hacking is when somebody gains access to a computer, typically without permission, to preform a certain task. Tasks range from using the computer as a distribution point for spam, downloading files...
Just watch swordfish, die hard 4 or war games. You'll understand what hacking is then... or at least the Hollywood perception. And that's why most people get into hacking anyway, by watching films...