Teething is the process during which your baby's teeth begin to break through the gums. Your child is born with all twenty of its milk teeth; you just can't see them, as they are submerged beneath its gums.
Cutting the first tooth is usually the most painful or uncomfortable for a child. Having said that, it can also be the emerging of the larger molars that can cause the most discomfort. You'll no doubt have noticed just how important the mouth is to your baby. More than any other part of the body, your child explores their world using its mouth. Even if the emerging tooth is not painful, it certainly will be a distraction for your baby and can cause the crankiness you see. As an adult you will know the feeling of annoyance when you get a new filling, bite the side of your mouth or simply have a piece of food stuck between your teeth. Now imagine how it must be to experience this for the first time; the feeling of distraction increased many fold.
Despite what you might think, teething doesn't stop when your child has their full set of milk teeth. From around the age of six the permanent (adult teeth) will begin to appear, and the symptoms - and problems - of teething may appear a second time round. First will appear the central incisors, then about a year later, the lateral incisors. At around nine to ten years, the first and second premolar (Bicuspid) will push through. Your child should have his canine teeth about the age of twelve.