-There are single core CPUs (traditional CPUs-Pentium 4, Athlon 64).
-Most popular and mainstream now are the multicore CPUs which branch out to dual core CPU and quad core CPU (with the latter being the most expensive type). ie, Pentium D, Athlon 64 X2, Intel Core Duo, Intel Core 2 Duo, Intel Core 2 Quad
AMD changed it's line of the Socket 939 Athlon 64 (once the #1 selling CPU and the best at its time), to the newer Socket AM2 Athlon 64 and Athlon 64 X2 (dual core setup) which I'm sorry to say, doesn't stack too well against Intel's new powerhouse--Intel Core 2 Duo (their version of the dual core CPU). Intel also has a Core 2 Quad CPU which has 4 cores, but that's pretty extreme and very, very high priced.
The Core runs so well and literally smokes AMD in virtually every single category, which is why they are the number one selling CPU at this stage. I firmly believe that it is the Conroe core that Intel introduced, that killed AMD. This chip is HIGHLY overclockable and rips through applications. But if you know AMD, they'll come up with something to combat Intel. It's an ongoing thing.....
If you really want to get a good CPU as of this moment, look for the Intel Core 2 Duo E6600 or higher. And if you really multitask running intense applications, the quad cores will just blow you away.
Intel Core 2 Duo E6600 Conroe 2.4GHz 4M shared L2 Cache LGA 775 Processor - Retail
Special savings w/ combo deal, limited time offer
3x Winner of Customer Choice Award - CPUs / Processors (I voted for this one before before anybody took notice. Everyone was all about the E6400, but I was into the E6600)