A heatsink is a metal thing that is used to transfer heat away from something hot. Primarily it's used on the cpu. The cpu gets really hot, so a heatsink is put on to help keep it cool. Unfortunatly, that is not enough. One also need to move the air for better cooling. So a fan is placed on the heatsink usually blowing air onto it. To make it work even better fans are placed on the computer case to create airflow. One fan usually blows air in and another fan sucks air out. Hot air rises, so you want the intake lower in the case and the outtake higher up. Fans and heatsinks, alone, cannot cause the temperature to drop below room temperature.

Common materials of heatsinks are copper and aluminum. Copper generally cools better. There is a thermal equation to tell specifically how it performs but I forget where to find it. The size of the heatsink, the number of fins, spacing of the fins, and shape all cause the heatsink to perform differently. Also other means can be used to cool the heatsink such as water or liquid nitrogen, often considered exotic.

You can find some pictures here.

Adding fans will not overclock your system, but it might help your overclocking success.