White Hat Hacking

What I know about hacking is that you can get hacked by getting your IP address tracked, and through various different viruses as well. I don't know what the term "White hat hacking" means exactly either.
Questions:
What do you gain from white hat hacking?
What is the purpose?
Who gets hacked the most?
Links:
http://www.secpoint.com/What-is-a-White-Hat.html
http://www.techradar.com/news/internet/tools-and-tricks-of-the-white-hat-hackers-515208
Bulletins:
-A white hat hacker can be defined as someone who is payed or asked to hack a certain program or anything on a computer, to find out the vulnerabilities to determine whether it is a fit program to use or to improve the hacked program.
-White hat hackers are also called "ethical hackers"
-The first released records of white hat hacking (for security purposes) was by the U.S Air Force, Who were testing out whether the Multics operating systems was safe enough to use as a top secret system. Their results were that albeit a better system than others, it had vulnerabilities in most of its security branches that could be found with little effort by the white hat hackers, thus normal hackers too.
-From this, 2 people were able to make a free download that had programs they used to describe how to get information from a source and thus exploit it, then how to avoid such attacks, and they called it SATAN.
-This is an employment as well.
How this relates:
This relates to me because all of the programs I have used to do with microsoft have most likely  been white hat hacked to find vulnerabilities and to improve the safety of the program. A question I still have is how do they find hackers to employ?