To fully appreciate why you see what you see on television stations in your market, it helps to understand the difference between an O&O and an affiliate station.
An O&O is a station owned and operated by the television network or its parent company. In the Washington market, for instance, WRC is owned by NBC and WTTG is owned by Fox.
An affiliate, by contrast, is owned by some other company that signs a contract with a network agreeing to run its programming for a set number of years. The Washington ABC station, WJLA, is not owned by ABC or its parent, Disney, but by Washington-based Allbritton; the market's CBS station, WUSA, is not owned by CBS or parent Viacom but by McLean-based Gannett.
The reason the networks do not own all of the stations that carry their programming is that the federal government restricts the percentage of the country that a network can reach with stations it owns.
Stations owned by the networks tend not to preempt network programming with the vim of affiliate stations. This is a good thing if you would rather see the next original episode of your favorite program than a locally produced documentary about Redskins cheerleaders. On the other hand, it may mean that someday when you are watching an O&O's local coverage of the sniper shootings that have left several dead, the station may inexplicably break away to bring you the network-owned "John Walsh Show."
Likewise, affiliate stations are more likely to preempt network news coverage of nationally significant events in your market to showcase their on-air news talent. This can make sense when the network has parachuted in one of its New York celebro-journalists to cover the event; less so when a correspondent from the network's local bureau, who lives in town and knows the local scene, is covering the news story.
And, for obvious reasons, O&Os tend not to preempt their boss's new Jimmy Kimmel-hosted late-night talk show.
In addition, O&Os are subject to calls from the Big Boys at Corporate that go like this: "Say, you're gonna have to move your very popular local morning news show, on which you get all the ad time, into a less desirable time slot because we've decided to super-size Katie Couric." Or: "Listen, the guys here at Twentieth Television have decided to try out this new daytime syndicated show 'Live Like a Star,' which we think could be the next big thing, so giddy up and look for a time slot on your lineup, okay?"