Void Sims (aka Ocean Sims or Empty Sims or Openspaces) are sims that (usually) contain only water and do not allow building. They exist primarily to separate normal sims, yet still allow physical travel between them.
Despite the fact that Linden void sims generally have building disabled, void sims can in fact support the feature, as well as dry land, just as a normal sim can. They simply have lower prim limitations (1875 instead of 15000) and probably more easily suffer performance degradation under heavy load.
The limitations of void sims require less server resources, such that four void sims can be run on the same hardware that one regular sim uses. In the past, void sims were run on older hardware that used to run regular sims, which became available after upgrades. On April 26, 2006, Torley Linden posted a summary of the current situation (forum link): void sims are now being installed using the same hardware as regular sims, at the aforementioned 4:1 ratio.
Estate owners can now (5/16/06) purchase voids in packages of four (4) and place them where they like around their estate.