Hackers

They do it for the same reasons people break into houses. Curiosity, boredom, criminal intent, etc. are all motivating factors.

You detect hackers by using forensic techniques and tools, such as investigating audit logs, security logs, etc.

You prevent hacking on your PC, by using firewalls, anti-spyware, and anti-software.

BFilmFan is correct. Here is some more information on Hackers:

From http://www.webopedia.com/TERM/H/hacker.html:
A slang term for a computer enthusiast, i.e., a person who enjoys learning programming languages and computer systems and can often be considered an expert on the subject(s). Among professional programmers, depending on how it used, the term can be either complimentary or derogatory, although it is developing an increasingly derogatory connotation. The pejorative sense of hacker is becoming more prominent largely because the popular press has coopted the term to refer to individuals who gain unauthorized access to computer systems for the purpose of stealing and corrupting data. Hackers, themselves, maintain that the proper term for such individuals is cracker.

Another place for more information on hackers is: http://tlc.discovery.com/convergence/hackers/hackers.html
The learning channel did a great story on this and provided the following about methods of dealing with hackers:
http://tlc.discovery.com/convergence/hackers/articles/method.html

Hackers hack for two underlying reasons:
1) Pleasure : hacker likes the challenge of 'Software's walls'.
2) Money : By discovering exploit, bugs, ... he can make money.