Check out this link for useful information about typical communication development and an explanation of the various speech/language impairments.
http://www.asha.org/public/speech/development/
Below is a link that gives a nice summary of what your child should be understanding as well as what he or she should be able to say at various stages of childhood development.
http://www.speech-language-therapy.com/index.php?option=com_content&view=article&id=34:ages&catid=11:admin&Itemid=117
https://www.asha.org/public/speech/development/suggestions/