White hat hacking

What I know : So far I know that it has to do with hacking .
Questions before reaserch:
1. How do you hack into someones computer ?
2. what is white hat hacking compared to normal hacking ?
3. will hacking give you viruses ?

Information :
http://searchsecurity.techtarget.com/definition/white-hat
http://www.secpoint.com/What-is-a-White-Hat.html
http://www.investopedia.com/financial-edge/0811/famous-white-hat-hackers.aspx#axzz27VTn3A1u

Thing I learnt about the topic:

• white hat hackers are good 
• white hat hackers are hired by companies to look for possible security breaches
• There are many white hat hackers that do it as a hobbies and notify people by e mail
• hacker means security breaker 
• it began when a security check was conducted by the us air force.

How it relates in my life: people are normally hacking in to social pages like Facebook and Hotmail to find information or create acts of vandalism but white hat hacking is very different because it is for a good reason , they help people find holes in their security systems. And many people hire them to check their computers. so it relates to my life because I might hire someone to check my computer for any security breaches.

Questions:
1. is it illegal to be a unregistered white hat hacker ?
2. what is the difference between hacker and cracker?