The basic technology for electronic television (as opposed to earlier mechanical efforts) was invented by Philo Farnsworth and Vladimir Zworykin around 1930, and the first sets became commercially available in the late 30's. Collectors draw a sharp distinction between these vintage pre-war and post-war television sets, as very few of the former were manufactured and only a few hundred still survive.
1946 through 1955 was the first big growth period for television sets. In 1947 there were only about 100,000 TV sets in the U.S. (many of them in bars and clubs) but by 1953 there were 13 million. CBS started broadcasting in color in 1951, and all-electronic color sets were first introduced in 1954. By far the largest television set manufacturer in those early days was RCA, but other vintage television brands include General Electric, Dumont, Andrea, and Zenith. RCA also made sets for Westinghouse and for Sears.
Most collectors collect black and white sets from 1939 to 1949 and color sets through 1960. For black and white sets made after 1949, the values drop off dramatically, with some exceptions. There were a lot more TVs produced after the mid 1950s, so there are many still around (and they started to become less visually interesting, with metal cabinets). There are also people who collect early transistor sets from the 1960s and 70s, and micro TV sets.