From The TV IV

Syndication happens in two ways when it comes to American TV: first-run and off-network.

[edit] First-run Syndication

First-run syndication refers to programs that run on individual stations as opposed to a particular network in their initial broadcast. This means that a show that is syndicated, like Jeopardy!, may run on an ABC station in one market but a CBS station in another. The nature of syndicated shows means that it will not be on the same time everywhere, as each individual station will choose when it airs.

[edit] Off-network Syndication

Off-network syndication refers to selling old episodes of a program to individual television stations. 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. Examples of this include ER, where reruns air on TNT while first-run episodes still run 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).