Whenever you see 24x, or 8x, don't you wish you understood what it meant? Well, here is an Explanation.
1x refers to a basic transfer of 150KB/s, and all speeds are increments of that same 150KB. 2x is 300KB/s, 4x (also called Quad-Speed), is 600KB/s, and so on. Here is a List, calculated for you, so you don't have to!
Also see DVD Reading and Writing Speeds.