Introduced in Unearthed Arcana and reproduced on the SRD, gestalt characters get "the best of both worlds" with whichever classes the player selects. Rogues with full arcane spellcasting, fighters with a full cleric spell list and any other combination are possible.
Character versatility is improved heavily with this rule in play. Even though players can select two classes, gestalt characters are not "twice as powerful" as normal characters. They still draw their HP from the same pool and still rely on their (better) class for limited skill points.
Gestalts can generally handle more and tougher encounters. This variant works particularly well if you have very few players in your group.
This tag should be used when asking questions that specifically relate to that variant. It should not be used as a related anecdote. If your problem or question exists wholly or in part because of the combination of classes, use this tag. If another problem or question is your focus and you're only mentioning the gestalt variant anecdotally, do not use this tag.