There's no point me explaining how just one join works. You might as well learn them all. Up until a few months ago I never understood how they worked but I knew which join to use at the right time. Take a look at this helpful article:
http://www.devshed.com/c/a/MySQL/Und...ing-SQL-Joins/
And once you understand the basis of SQL joins you can look at how they work in MySQL in this other tutorial:
http://www.devshed.com/c/a/MySQL/MySQL-Table-Joins/