What are the stages of child speech and language development and why?

The acquisition of phonology is fairly well studied for English. And although, there is a lot of variation among individuals and there are no definite sequences or ages of acquisition, you can find fairly reliable tendencies across the population. However, these may not be very useful (to a lay person or even a linguist not trained in speech therapy) trying to determine whether any individual child is developing typically or there is cause for concern.

Any list of these stages would be too long for here but you can find it in any introductory text on linguistics that covers language acquisition (partly because it is the one area of language acquisition where there is some certainty). This Wikipedia article does a very good job of describing some of the key phases.