What Causes Tooth Decay?
Bacteria, if left around or in between the teeth, will secret acids that begin to slowly eat holes in the teeth, eventually leading to tooth decay.
Tooth Parts -- Knowing something about the teeth helps understanding how to take care of them.
Enamel
The hard outer surface of the teeth is called enamel, it is like rock.  
Tooth bacteria can't eat enamel.  The acids they secrete can eat thru enamel.
It takes about two years or more of not flossing for the bacteria's acids to eat through enamel.
This process is greatly sped up by drinking lot's of soda pop.
The acids in pop are similar to the acids that dentists use to rough the teeth up before bonding a filling to the tooth.  Only the dentist etching process only lasts 30 seconds or so, not repeated exposures of 10 or more minutes in time day after day.
Dentin
Underneath the enamel is a hard bone like tooth structure called dentin. Bacteria can eat dentin.  This is called tooth decay.
If the decay is large, it can get into the pulp.
Pulp
Inside the dentin is the pulp (or nerve and blood vessel) of the tooth.  To bacteria, the pulp is like dessert.
If the bacteria decay the tooth far enough to reach the nerve, the tooth will ABCs.
An abscesses (infected) tooth needs either a root canal or extraction.
If pressure from the infection builds up, an abscessed tooth can become very painful and possibly life threatening.  
If you think you have an abscessed tooth, call the office, and get it looked at and treated as soon as possible.