In addition to infinity's excellent instructions above, there are two more commands you might want to know about:

12. Type "stat" to get a simple message of how many messages you have.
13. Type "top x y" to get back the first y lines of message x. This is unbelievably useful in that you can say something like "top 1 0", and get the first zero lines of message 1-- in other words, only the headers. This is how you see who the message is from without reading it. After using top x 0 you are then free to decide whether to dele the message or retr it.

Once you get used to this, you begin to like it a lot-- checking your email from anywhere without having to find and set up a mail program gets really nice. If you want to see the full guide to the protocol by hand, it's all described nicely in the RFC..