Association for Computing Machinery
The Association for Computing Machinery, or ACM, was founded in 1947 as the world's first scientific and educational computing society. Its membership is currently around 75,000. Its headquarters are in New York, New York.
ACM is organized into over 170 local chapters and 34 special interest groups (SIGs), through which it conducts most of its activities.
Many of the SIGs, like SIGGRAPH and SIGCOMM, sponsor regular conferences which have become famous as the dominant venue for presenting new innovations in certain fields. The SIGs also publish a large number of specialized journals, magazines, and newsletters.
ACM also sponsors other computer science related events such as the worldwide ACM International Collegiate Programming Contest (ICPC).
ACM Press publishes a prestigious academic journal, Journal of the ACM, and general magazines for computer professionals, Communications of the ACM and Queue. Many of the great debates in the history of computing have taken place in the pages of Communications. Examples include the famous "GOTO is bad" letter, the issue of what to call the then-fledgling field of computer science, and the issue of changing ACM's name (since the "machinery" in question is no longer the size of a house and is now measured in micrometres). All three attempts at changing ACM's name have failed.
ACM's primary competitor is the IEEE Computer Society. It is difficult to generalize accurately about the distinction between the two, but ACM focuses on theoretical computer science and end-user applications while IEEE focuses more on hardware and standardization issues. Another blunt way to state the difference is that ACM is for computer science majors and IEEE is for electrical engineering majors.
Of course, there is significant overlap between the two organizations, and they occasionally cooperate on projects like developing computer science curricula.
For more information about ACM, see http://www.acm.org/.
For more information about the ACM ICPC, see http://icpc.baylor.edu/icpc/.