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 mytable.

| Field     | Type    | Null | Key | Default | Extra |
| courseid  | int(11) | YES  | MUL | NULL    |       | 
| dept      | char(3) | YES  |     | NULL    |       | 
| coursenum | char(4) | YES  |     | NULL    |       | 


It means that the field is (part of) a non-unique index. You can issue

show create table <table>;

To see more information about the table structure.