No matter how smooth and flat the surface of the heatsink is, and no matter how smooth and flat the surface of the CPU is, there will always be tiny microscopic air pockets between two solid objects such as these. These air pockets can trap heat and impair heat transfer from the Processor to the HSF. Thermal paste fills in those gaps.
Thermal grease can just be this sort of sticky white...stuff..or it can contain silver, the latter of which will transfer heat a lot more effectively. As F1N3ST suggested, Arctic Silver 5 has silver in it (thus the name), and you will likely notice quite a substantial decrease in temperature when you get it.
