A concept much abused by politicians, market researchers and other necessary evils, but really it's very simple: a community is a group of people with common interests, which recognises and acts upon those interests. Otherwise it's just a random group of people. Communities have historically been defined by geographical location, ethnicity, religion and so on; actually communities can be based on anything from sexual preference to consumer identification to operating system. The key criterion is: if it thinks of itself as a community, and acts like a community, then it probably is one.