There are many different sign languages depending on your country, etc. In the US is only one true sign language, ASL, but a few different sign language systems. The true languages are a language completely on their own, whereas systems are a visual representation of spoken languages (i.e. SEE, signing exact English, is a visual representation of English rather than anything separate). Even if you don't know any deaf, Deaf, or hard-of-hearing people, sign language can be extremely useful in early communication as children are capable of understanding and using sign long before they have the ability to form spoken words. Other than those basics...it would fill up many, many books to continue the explanation :).