Heat is your computer's worst enemy. Excessive heat will cause random errors in your CPU, memory, motherboard chipset, video and everything else inside your computer.
All computers generate heat in the course of their normal operation.
If this heat hangs around in the case, the computer overheats.
The computer gets rid of this heat by transfering it to the air in the case and then blowing that heated air out the back of the case.
If the heated air can't get out, your computer case turns into an oven. This is not good.
The first effects of overheating are seemingly random crashes. If your screen goes blue with a lot of cryptic numbers on it, or if the screen goes black, or if your programs simply go wild, you've crashed. On a more technical level, these are due to random errors in your memory, CPU, video and nearly everything else. This stage of overheating is easily reversed - just restore the lost airflow, and your computer returns to normal.
The second stage of overheating occurs when the innards (that's a highly technical term :-) of the chips start to melt. This damage is not reversible. Once melted, chips and other components must be replaced.
The major causes of overheating are dust, fan failure, and improper case locations. More about those subjects in later articles.