There are two particular SELECT keywords that will be of particular use to you: DISTINCT and AS.
The DISTINCT keyword is used in SELECT statements to remove duplicate result rows. Note that DISTINCT matches all attributes that you have selected - that is, if you select FirstName, LastName, and Age, then all three must be the same for a match to be made. If you wanted to find all distinct last names in your table, you would use "SELECT DISTINCT LastName FROM usertable;".
Normally when you SELECT records from your table, they are put into the PHP associative array with keys named after the field names. When you use AS, MySQL lets you change the field names (temporarily) to something that is more useful to you. Taking from our previous CONCAT example, you could use AS like this: "SELECT CONCAT(FirstName, ' ', LastName) AS FullName FROM usertable;"
When you get the query results back, you will not have FirstName or LastName set, or the horrible "CONCAT(FirstName, ' ', LastName)". Instead, you will find your array has the "FullName" element set, which is much easier to read and handle.