From The TV IV
Off-network syndication refers to selling old episodes (reruns) of a program to individual television stations, or in some cases a cable network. After a first run show has lasted long enough and become popular enough (typically airing 100 episodes is the standard), its back catalog will be sold, either to a network affiliate or independent station in every market, or to a cable network. An example of this includes ER, where reruns aired on TNT while first-run episodes were still running on NBC.
While the networks generally take most of the profits from first-run episodes, syndication is often very lucrative for those involved with the production of the show (Larry David reportedly sold his share of the syndication rights to Seinfeld for US$200 million).