The National Institute on Deafness and Other Communication Disorders has some very informative fact sheets and guidelines that describe typical and atypical speech and communication development. One great page on infant and toddler communication milestones is located at http://www.nidcd.nih.gov/health/voice/speechandlanguage.asp#mychild. At this link, you can answer a series of questions in an interactive checklist to help you to determine if your child is on track.
Aside from these useful fact sheets, there is also some very interesting statistical information on this site. For example:
The prevalence of speech sound disorder in young children is 8 to 9 percent. By the first grade, roughly 5 percent of children have noticeable speech disorders; the majority of these speech disorders have no known cause.
I recommend this site for parents even if you do not have any concerns about your infant or child's development, as it will help you to understand what some fellow parents and some of your child's classmates may be going through.