The term television channel generally refers to either a television station or its cable/satellite counterpart (both outlined below).
Sometimes it is confused with the term television network, which (when properly utilized) describes a group of geographically-distributed television stations that share affiliation/ownership and some or all of their programming with one another.

The Big Three Television Networks are the three traditional broadcast (over the air) TV networks in the United States: ABC, CBS and NBC.
NBC and CBS were founded as radio networks in the 1920s. They gradually began experimental television stations in the 1940s.
ABC was spun off from NBC in 1943 when the U.S government found evidence indicating NBC was operating a monopoly. huh.

American Broadcasting Company (the last of the Big Three broadcasting networks to use the full name)-- sometimes referred to as the Alphabet Network and uses the 'abc' lettering as its logo. ABC is now owned by The Walt Disney Company.

CBS (an abbreviation for Columbia Broadcasting System, its former legal name) -- sometimes referred to as the Tiffany Network; the nickname is either a reference to the perceived classiness of CBS programming, or an allusion to some of the first demonstrations of color television, held by CBS in a former Tiffany building in New York City, in 1950. It is also sometimes referred to as the Eye Network after its eye logo. The network was previously owned by the media conglomerate Viacom (itself once a subsidiary of CBS). It and other traditional broadcasting assets became the new CBS Corporation following the split of Viacom at the end of 2005.

NBC (an abbreviation for National Broadcasting Company, its former corporate name) -- sometimes referred to as the Peacock Network due to its stylized peacock logo. The network is now part of the media conglomerate NBC Universal and supplies programming to more than 200 affiliated U.S. stations. NBC Universal is a unit of General Electric (GE).

The prohibitive cost of starting a broadcast network, coupled with active sabotage by the Big Three Networks (particularly NBC and CBS), led to the downfall of almost all new companies. A viable 'fourth network' would not become competitive with the Big Three until the Fox Network was founded in 1986.
Fox survived where DuMont and other previous attempts to start a fourth network failed in part because Fox programmed just under the number of hours to be legally considered a network by the FCC. This allowed Fox to make money in ways forbidden to the established networks, since during its first years it was considered to be merely a large group of stations.