hyperthreading

Hyperthreading takes advantage of additional registers--circuits that help manage data inside a chip--that come on existing Pentium 4's but aren't used. Through these registers, the processor can handle more tasks at once by taking better advantage of its own resources. The chip can direct instructions from one application on its floating-point unit, which is where the heavy math is done, and run parts of another application through its integer unit.

in simple terms...

it is 1 phsical processor and 2 logical (technically 1.5)
so...
the computer see's 2 processors and you dont! hyperhreading hasnt really been optimized in many applications or games yet tho...