The Doctor is a character in Star Trek: Voyager. He is an Emergency Medical Hologram based on the appearance and personality of his creator, the engineer Doctor Lewis Zimmerman. He was created to be an emergency supplement, but when Voyager becomes stranded in the Delta Quadrant (70,000 light-years from Earth) with their chief medical officer dead, he is activated to fill the role. His character develops throughout the series as he makes friends, expands his programming, and gains a mobile emitter that allows him access to the entire ship (previously, he had been confined to Sickbay).
It is worth noting that the Doctor has several hundred "cousins"—other EMHs who were decommissioned and set to work as laborers.