I'm no expert, but here's the best explanation I have.

GET and POST are both methods of passing information from one page (usually a form) to another. They differ in how they do it.

GET does it by including the information in the URL itself. When you see URLS with "?" and "&" in them, those are usually GET variables being passed.

POST does it in a less visible way, by somehow including them in the actual HTTP request or something. They get sent "under the radar" and are invisible to the user (at least, they are not in the URL). Because of this, POST is usually preferred since it's a little more private/secure.