hoob:
DNA is like a string of protein recipes. The chain of molecules in a gene [recipe] is used to build a protein.
Think of a cooking recipe. You take certain QUANTITIES of certain INGREDIENTS doing certain MIXING, HEATING, COOLING etc. Mutate any of these aspects (change quantity, change an ingredient, duplicate a step, reverse something) and you are going to get a new result. Accumulate several of these and you most certainly have a new recipe. Mutations to DNA change the resulting proteins in the same way.
EVERYONE: If you haven't already, go and read the link JANH posted on page one of this god forsaken thread. Fascinating stuff.