The Process of Decay
Dental decay is a disease.  The mouth has lots and lots of germs that live in the plaque and saliva in your mouth.  Yuck!  That bacteria is acidic and will eat through the enamel coating the crowns of your teeth (the white shiny part above the gum).
The plaque becomes more acidic when carbohydrates are introduced into the oral cavity.  The acid level  stays high for approximately 20 minutes and then the saliva dilutes it back down to normal. 
That's great if you don't eat sugary foods very often and have plenty of saliva.  Well, what if you snack frequently on candies or sodas and your acid level stays high for long periods of time?  Or, what if you don't have lots of saliva because of medical problems or medications?  The result is weakened enamel which will  demineralize (become weak) and then turn into a cavity.  Then the only solution is to remove the decay and fill the cleaned tooth.
The good news is that limiting snacks high in sugar and thorough dental hygiene can help lesson the risk of decay.
The other good news is that are other steps you can take to help prevent cavities!  Please see Save Your Enamel and we'll discuss it.