After roasting, the beans need to be ground.  Ground coffee has a greater surface area than the whole bean, so hot water can get to it more quickly and release the flavours. However, excessive grinding will cause the oils to evaporate and the coffee to taste bitter and acidic.

Grinding tips
Never grind more coffee than you will use for immediate brewing. Once the beans are ground, the flavourful oils are exposed to the damaging air. As these oils dissipate, so will the flavour of your coffee. Once ground, coffee will begin to lose its flavour almost immediately. 
 
Different methods of brewing will require different grind consistencies.  It is important to have the correct grind for your coffee maker, because without it your cup of coffee will not have the proper taste.  Here is a guide to help you:

Method                        Grind needed 
 
Turkish                         Pot  Pulverised
Filter Machine                Very fine grind
Espresso Machine           Fine grind but not too powdery
Jug Method                    Medium grind or a bit coarser 
Cafetière                       Medium ground
Mocha Pot (Stove Top)    Fine grind
Original Percolator          Medium grind
Vacuum Method              Medium grind, or a bit coarser