"DDR, DDR2, and DDR3 memories are classified according to the maximum speed at which they can work, as well as their timings. Timings are numbers such as 3-4-4-8, 5-5-5-15, 7-7-7-21, or 9-9-9-24, the lower the better. In this tutorial, we will explain exactly what each one of these numbers mean."
Understanding RAM Timings
Posted on Wednesday, May 18 2011 @ 20:55 CEST by Thomas De Maesschalck
Hardware Secrets learns you in detail what numbers like 7-7-7-21, 8-8-8-24, or 9-9-9-24 after the memory specification mean. You can read it over here.