American Sign Language is a language used by many people in the US who cannot hear (see Deaf, deaf and Hard of Hearing).
Many people believe that only Deaf people use ASL (or other sign languages), but many hard of hearing people also use sign language to supplement what they can hear among friends or family ... or even become more proficient and can use ASL with Deaf friends or through an ASL Interpreter.
ASL uses hand shapes, positions, movements, facial expressions, and body movements to convey meaning. ASL uses an alphabet (finger spelling), sign representing ideas, and gestures. ASL is an independent language that has its own grammar and syntax; it is not simply a manual version of English (see Signed Exact English SEE and Pidgin Signed English PSE).
Sign languages vary from country to country in exactly the same way that spoken languages vary from country to country.