I've heard that 10 hairs together is a curl and 10 hairs apart is frizz. I think that's pretty accurate. Frizz is really the absense of curl formation, and it can be caused by humidity, poor application (or no application) of styling products and other factors. If your hair is in good condition, and you use products with the right amount of hold and you apply them in sections, you should be able to reduce frizz significantly.
Frizz is your hair reaching out for mopisture, like a tree sends out root's, your hair sends out frizz, or humidity attacks :)