Sign language is NOT a lesser language.
It has its own highly-structured grammar and is able to convey complex concepts.
Many countries have their own sign language, and these can vary slightly according to region, just like the accents and slang of spoken languages. American Sign Language is often referred to as "ASL," and is the major signed language in the United States. Americans can also use "signed English", which translates into spoken English more directly than ASL.