American Television Network Affiliation
Television networks are handled and broadcast differently in the US. One major company 'The Network' (There are 8 main commercial networks, the biggest four being ABC, CBS, NBC, and Fox. The smaller 5 are WB, UPN, PAX and PBS) is responsible for buying and in some cases producing programming, but does not broadcast it directly.
Instead, the network relies on local stations in all the different regions (or 'markets') to distribute the programming to the population. These are called 'affiliates'
Affiliate stations hand over control to the networks at certain times, notably mornings and peak time (called primetime in the US), as well as afternoon lineups of soap operas. The networks handle scheduling during these time periods; if a show gets pulled, it is across all stations of the network. During these periods, programming is branded under the network name only, although in most cases the local stations do put their own logos on the screen coming back from break or during network promotions.
When the affiliates are not broadcasting network programming, their own lineups consist of home-grown output, mostly local news, of which there is a LOT more of than in the UK, or syndicated programming.
Syndicated programming is similar to networked programming, although the distribution is not handled by the network and the local station has control of where to put it.
Affiliates in the major cities/markets are owned by the networks and are called 'Owned and Operated' stations - or O&Os. But for the most part, this is the exception to the rule - most stations are owned by independent companies.
Generally, branding of affiliate stations nowadays is network/channel number. You see a lot of ABC 7s, NBC 4s, and CBS 2s, among the O&O station groups, but there are still a lot of affiliates that go with just the call letter identifications "WUSA 9", "KARE 11", even a "catchy" name! (WXIA in Atlanta is "11 Alive"), some even just use a channel number.
We believe Fox that all affiliates have to use Fox-approved names and channel logos, which is why all the Fox affiliates look exactly the same.
Pax is different, identification on Pax's affiliates is usually just "PAX" with an ID at the top of the hour. Some Pax affiliates, however, have tried local news and do have different, separate identifications. Scheduling is controlled by Pax themselves; this is more like ITV now.
PBS, even though it gets government funding like the BBC, is more like how ITV used to be. Less a network than a collection of separate affiliates who produce the programming that is shipped to PBS and then "networked" to the local stations. They even used to have frontcaps until recently. Some PBS affiliates embrace the national PBS brand and graphical look. Some, however, go their own way, such as WHYY in Philadelphia, which emphasises their own look over PBS'.