Ars gives a Hyperthreading lesson
Hyperthreading has been getting a lot of coverage lately, and Ars Technica is getting in on the fun
Hyper-threading's strength is that it allows the scheduling logic maximum flexibility to fill execution slots, thereby making more efficient use of available execution resources by keeping the execution core busier. If you compare the SMP diagram with the hyper-threading diagram, you can see that the same amount of work gets done in both systems, but the hyper-threaded system uses a fraction of the resources and has a fraction of the waste of the SMP system; note the scarcity of empty execution slots in the hyper-threaded machine versus the SMP machine.
SMP and Superthreading also make appearances in the article, which should give you a great theoretical base to draw from when we start looking at benchmarks of Intel's new Pentium 4 in action. It's probably a good idea to spend a little quality time with the article rather than just skimming over it while you down your morning coffee; there's a lot of great stuff in there you don't want to miss.