Hyperthreading
Hyperthreading is an innovation made by Intel designed to facilitate and assist the execution of multiple threads. For most consumer applications this translates to an improved multitasking experience however marketers tend to like to "suggest" that it is like having twice the processing power -- which it is not. It should also be noted that Hyperthreading really only shines through when multiple CPU-intensive tasks are executing simultaneously.
Dual/Multi Core
A recent craze, Dual and multi core processors are, for all intents and purposes, "two processors inside one convenient package". Each processor will have independent access to cache and stackspace; by having multiple cores the processor as a whole can deal with multiple threads of execution (i.e., multiple applications, multiple instructions etc) without any [significant/measureable] overhead (i.e., Dual Core processors are what marketers hype-up HyperThreading to be)