Selling the right to broadcast a television or radio program to independent stations, syndication has enabled the modern system of 24/7 broadcasting by providing producers of content with a consistent revenue stream, and stations with enough programming to satisfy their eager audiences.
There are a variety of syndication types, including movie packages and public broadcasting, although perhaps the two most common, and lucrative, are first-run and off-network (re-runs).
First-run syndication refers to shows that have never been aired previously and common examples today include Wheel of Fortune and Judge Judy.
Off-network syndication refers to shows that were first aired on network television, and are being broadcast again (aka: re-run); common examples today include The Big Bang Theory and Modern Family. Note that when these programs were first aired by the networks, they were shown through either stations that were owned-and-operated by the network (O&O) or network affiliates, with which the network has a special contractual agreement that covers programming as well as other issues.