What follows is not based on a thorough study of many babies, but rather on more casual but long-standing observations of three:

The first form of communication by babies is binary: crying ('need attention') or not. After that they add a bit to communicate happiness by smiling. The next bits tends to be asking for a particular person ('Mamamama' or arms stretched toward Daddy). After that, they often develop a sign for 'I want' (food, etc.). And then, around 11 months, comes the all-powerful but more abstract Yes and No, often in the form of a nod or a nay. Babies seem to show a subtle evolution in their understanding of their meaning. It seems to start as describing what baby is allowed and not allowed to do --what will elicit approval or disapproval from parents. Yet initially a baby can switch rapidly between Yes and No, making it unclear whether he/she truly understands their meaning. Then come signs associated with specific powerful sensory experiences (such as a symbol for an airplane flying overhead).