An expansion is additional software (usually made available for retail sale) that adds new content and features to a game.
The content of an expansion is usually not available to anyone who does not own the expansion. Most expansions will not install or run as stand-alone products.
It was stated on an interview before the first expansion (possibly by Rob Pardo) that "We have made concepts for this expansion, and we already have enough ideas for 5-6 more".
Blizzard has stated it plans to release one expansion per year for World of Warcraft. Yet development of new expansions does seem to take significantly longer.
There is an entry for popular ideas about future expansions.