Description
What To Look For
Recommendations

Already know what a Case is and what to look for?  Then skip to the Recommendations. 

Description:

The computer's case serves several functions.  The motherboard is bolted down to the case so that the case protects it and all other components.  The metal in the case also serves to ground the motherboard. The case's power supply converts power into a form the motherboard can use.

A good case should have ample expansion bays to be able to add additional internal and external devices. It should have a strong enough power supply to power all the components you plan to add to your computer. The case should be designed aerodynamically so that airflow will flow in through the front and out through the back to properly dissipate all hot air.  The case also needs to be sturdy enough to prevent components from moving around.

More Info:
What To Look For
Recommendations