LinuxCNC it the "home of users of the Enhanced Machine Controller - EMC". There's everything about the EMC there - definitions, howtos, configurations, helps. There's this wonderful users manual (PDF) which I am reading now, and has all the vital information for understanding how the EMC works. Here are some first interesting points that I noted, but if you want to know (much) more, read the whole manual.
The Enhanced Machine Controller (EMC2) can control machine tools, robots, or other automated devices. It can control servo motors, stepper motors, relays, and other devices related to machine tools.