Syncom 3 was the third attempt by Nasa to launch a geostationary satellite, and the first to be successful. Syncoms 1 and 2 had both failed -- contact was lost with Syncom 1 before it managed to lock into the correct orbit, and Syncom 2's orbit was geosynchronous but at an inclination that meant it was not fully geostationary -- that is, from Earth it appears not to move, as it orbits at a fixed point relative to us. Syncom 3's 19 August 1964 launch saw it take up residence at a point near the International Date Line.
Geostationary orbits had been proposed by both the Austro-Hungarian scientist Herman Potočnik and the science fiction author George Olive Smith in the 1920s and 30s, but it was Arthur C Clarke who popularised the idea. He proposed a system of four satellites that could, theoretically, blanket the entire Earth in TV and radio signals.
Today there are hundreds of satellites at different geostationary orbits, with functions ranging from weather monitoring, to communications, to spying.