Okay, the numbers associated with AMD processors are the rough equivalents to Pentium speeds. Though some might argue this (and I certainly don't have the means to physically test it), the number equates the AMD processor to the same speed (in megahertz) Pentium processor. For example, an AMD Athlon 64 3000+ would process about the same amount of data as a 3.0GHz Pentium.
AMD's processors work harder than Intel's, and back when the only thing people looked at was clock speed, AMD decided to add these numbers to show people that even though AMDs are physically slower, they are actually better. It seems to have worked, too. The standard is still used today, years later.