American Sign Language (ASL) is a visual language. With signing, the brain processes linguistic information through the eyes. The shape, placement, and movement of the hands, as well as facial expressions and body movements, all play important parts in conveying information.
Sign language is not a universal language -- each country has its own sign language, and regions have dialects, much like the many languages spoken all over the world. Like any spoken language, ASL is a language with its own unique rules of grammar and syntax. Like all languages, ASL is a living language that grows and changes over time.
ASL is used predominantly in the United States and in many parts of Canada. ASL is accepted by many high schools, colleges, and universities in fulfillment of modern and “foreign” language academic degree requirements across the United States.
-National Association of the Deaf, http://www.nad.org/issues/american-sign-language/what-is-asl
You will find wonderful interactive resources for language enthusiasts, ASL students and learners, instructors and teachers, sign language interpreters, homeschoolers, parents, and professionals who are interested in learning how to sign language online and/or beyond classes for practice or self-study listed here.
American Sign Language Library
American Sign Language Teachers Association
Classifiers in Sign Language
Health Education in ASL
Texas Math Sign Language