The pentatonic scale works over practically anything. You can use it in rock, metal, blues, whatever. There are minor and major pentatonic scales, but the minor scale in used much more extensively. When people speak of the pentatonic scale, it is the minor version which they are referring to. The pentatonic minor scale is the most simple scale. It is a substrate of the minor scale, which, in turn, is derived from the chromatic scale. Start with one of the pentatonic forms, and learn it really well. From there you can make cool riffs and even put together a solo.
This is the most common form:

A minor pentatonic:
-------------------------------------5---8---
------------------------------5---8----------
-----------------------5---7-----------------
----------------5---7------------------------
---------5---7-------------------------------
--5---8--------------------------------------