Speech language pathologists often introduce sign language to children who are "late talkers" as a way to help them communicate (and reduce frustration for the child and parent). The child will use the signs until he or she begins talking, and then will stop using them.
The following websites have pictures and videos of specific signs.
For basic signs http://www.babysignlanguage.com/dictionary/
More complex signs http://www.signingsavvy.com/browse-letter