Domain. The domain is the area of knowledge that interests the community. In free software, the domain is usually a particular technical problem that needs to be solved.

Community. The community is the set of people who care enough about the domain to give their own time to participate. In free software, even though domain may be very specific, interested community members can come from anywhere that's connected to the Internet -- which is one of the factors that makes the community software development model so powerful.

Practice. The practice is the way that work is done, by the community, to further their goals in regard to the domain. All frameworks, tools, ideas, stories, documents, legal entities, code, and so forth, are all part of the practice. It's the work, and all the tools used to get the work done.