This is a good question Andrew and it is a long answer. Basicly, a balk is an attempt by the pitcher to deceive a base runner. Here are some links that will try to explain this as I don't want to type out the whole rule book :

http://www.webball.com/cms/page2024.cfm

http://brainz.org/what-balk-baseball/

http://answers.yahoo.com/question/index?qid=20100412124138AAy0Mms

The last one is the best. Let me know if this helps you.

ALE,
Typically a balk is called when a pitcher makes a pitching movement toward the plate and then throws to a base.

http://en.wikipedia.org/wiki/Balk

wen u stop ur move ment to the plate

a balk is when a pitcher makes a movement before he is going to pitch the ball