WABC, WCBS and WNBC are the ABC, CBS and NBC O&O television stations in New York City. Conversely, KABC, KCBS and KNBC are the networks' stations in Los Angeles.
Call letters are necessary because channel numbers are duplicated dozens of times across the country. In the case of the New York and Los Angeles stations, the ABC stations are on channel 7, the CBS stations are on channel 2 and the NBC stations are on channel 4.
Generally, stations with call letters that start with "w" are east of the Mississippi River, while those beginning with "k" are west of the river. There are some notable exceptions -- KYW is in Philadelphia, while WFAA is in Dallas.
In years past, some companies owned radio stations and a television station in the same city which shared call letters. WSB AM-FM-TV in Atlanta, Georgia, is one example which survives today.
While call letters are the official identifier of a radio or tv station, the stations can choose whether to emphasize their call letters on the air, or something else. A radio station may call itself "Rock 102" as a way to establish its brand, but is still required to say its call letters on the air regularly.