SQL keys, MUL vs PRI vs UNI

What is the difference between MUL, PRI and UNI in MySQL? I'm working on a MySQL query, using the command: desc mytable; One of the fields is shown as being a MUL key, others show up as UNI or PRI. I know that if a key is PRI, only one record per table can be associated with that key. If a key is MUL, does that mean that there could be more than one associated record? Here's the response of mytabl