Different theorists have come to different conclusions concerning how exactly children develop across the various developmental channels. Some theorists believe that children develop smoothly and continuously, but other theorists believe that children develop more discretely in a series of stages, each of which is fairly stable.
Theorists who believe children grow continuously believe that kids constantly add new lessons and skills on top of old lessons and skills as they get older. They believe that children grow at a steady, uniform speed. Even though parents can't see it with their eyes, children are growing all the time right in front of them. Their bodies make new cells. Their minds learn new skills as they play and interact with other people every day.
On the other hand, theorists who believe children grow discontinuously believe children grow in stages as they seem to develop chunks of abilities and to experience events at certain times in life. To some parents, it may seem that their children learn to do things all of the sudden, like when a baby goes from only being able to crawl to being able to toddle around on two feet almost over night. Or, parents of young teenagers may say that they were amazed how their children went from thinking that kids of the opposite sex had "cooties" to constantly daydreaming about them. It seems as if these kids are growing lots in spurts at special times and then are not growing so fast for a while in between the spurts.
Both camps, continuous development and staged development, are correct in its own way, of course. While it is true that development is a continuous process that never stops, it is also true that there are stages to growth and that developments unfold at predictable times across the life span. The real difference between the two camps is likely the degree of magnification that each applies to its study, with the stage theorists taking a more distant but broader stance and the continuous theorists viewing things from up close.
This document and the following documents in this series will present child development as though it happens in stages. By thinking about stages, child development can be summarized in general groupings that can be more easily understood.