Hyperthreading

Hyper-threading is using one processor but logically dividing it into two so that it gives the user the benefit of two processors with only using the resources equivalent to almost one. This is achieved by sharing, partitioning and duplicating the various resources almost into two processors. Used by the latest Pentium processors, which are HT enabled, in layman's terms, it allows you to use more than two applications at the same time without slowing down processing speed.

Read this first: http://arstechnica.com/features/2002/10/hyperthreading

Hyper-threading is a technology which allows a single CPU to appear to be made up of two virtual CPUs. The CPU still only contains one core processor but has more than one set of registers. As far as software is concerned, there are two CPUs, but there is only one physical core.