For questions about homebrewing new rules content for an existing system, this tag should be used when locally-created, non-published content is at the heart of the question. (Note that soliciting ideas for creating homebrew is off topic for the site.) Not to be confused with the [house-rules] tag. For creating entire new games, use [game-design].
Homebrew, so called by analogy to home-brewing beer, is the creation of new game material by fans (not the publisher or licensed third party publishers) for an existing game system. This tag is for questions about creating new rules material; for creating entirely new games, use game-design.
Homebrew is for non-published content. Feats, classes, abilities, perks, traits, talents, or any other mechanical component not published by the holder of the license, or appropriately authorized third parties, constitutes homebrew, whether it's written by the group in question or some random person on the internet.
Homebrew is distinct from house-rules in that house rules are usually small ad hoc modifications of an existing system, while homebrew for an existing system is the creation of entire new subsystems or of rules content like classes, creatures, spells, equipment, and similar game material.