It depends on the age of the child. There are many misconceptions about our children's teeth. Let's clear up some of them!
In the beginning, and I do mean in the beginning, our teeth are tiny little buds in the third trimester of pregnancy. If Mom has a high fever, or has to use an abundance of erythomycin (an antibiotic) these little buds can become flawed. If these events happen, there is not much we can do to prevent the flaws. They can appear as white patches on the enamel of the teeth or yellow softer spots. This does not affect adult teeth unless the same things happen to the child at certain ages while the adult teeth are forming.
Your baby's teeth will begin to appear around 3 months, but not always. My first two little girls were around a year old. Yikes, baby dentures! 
Usually, subsequent teeth will erupt every two to six months until the total of these first teeth is 28. They will include the incisors (those are the teeth used to bite into things like apples.) There will be 4 on the top (maxilla) and four on the bottom (mandible. Next in the arch will be premolars, then molars. Here is where there is often confusion.
Your child will have most of his teeth by age three, even the child who takes longer. Around age 6, an adult molar will erupt. This will be behind the baby molars in the arch. It is NOT a baby tooth. It is a forever tooth, as is the second molar around age 12. By this time, most of the baby teeth are lost, but not in everyone.
When the first canine tooth is lost, this space can stay empty for a long time, causing anxiety in both child and parent. This is normal. The adult canine is very strong and very slow to erupt. X-rays of the area can provide relief if you just can't stand the wait.
Another name for the first teeth are deciduous teeth. This is easy to remember if you know that some trees, like oak and maple are called deciduous trees because they lose their leaves.
Around 5, the first baby teeth may get loose and are usually the bottom two central incisors. As a rule, the mandibular teeth are lost first and maxillary teeth follow. Is this always true? No, so don't worry if the progression is a little different. Your dentist is always happy to refresh you on these things.
It is very important to understand that the baby molars are worth restoring if a cavity occurs. Yes, they will be lost, but not until age 10 - 14. They hold an important place until adult teeth erupt. Money spend on baby molars often mean less orthodonic money will be spent later.
Sometimes, teeth are missing. This is congenital, because for some reason they did not form. Usually it's just a quirk of nature and nothing anyone did to cause it. In some families, it is inherited.
Your dentist is the person who can answer your questions and ease your fears. Your job? A big one! Teach your children to brush often and correctly. Anyone under 6, should have Mom or Dad's help every other day or so, just to make sure they are adequately cleaned. Beautiful smiles result! 
