From http://www.acm.org/:
ACM, the world’s largest educational and scientific computing society, delivers resources that advance computing as a science and a profession. ACM provides the computing field's premier Digital Library and serves its members and the computing profession with leading-edge publications, conferences, and career resources.
From Wikipedia: Association for Computing Machinery:
Association for Computing Machinery, or ACM, is a learned society for computing. It was founded in 1947 as the world's first scientific and educational computing society. Its membership is more than 92,000 as of 2009. Its headquarters are in New York City.
ACM is organized into over 170 local chapters and 35 Special Interest Groups (SIGs), through which it conducts most of its activities. Additionally, there are over 500 college and university chapters