The Caretaker was the name given to the Nacene entity who assumed responsibility for protecting the Ocampa. Some also called him "Banjo Man", since he communicated with Crewmembers from the USS Voyager through an illusion in the form of an elderly Human playing a banjo. (VOY: "Caretaker", "Projections")
After his people accidentally caused the destruction of the ecosystem of the Ocampa homeworld sometime around the 9th century (c.f. "The Warming"), the Caretaker was selected to remain behind with his mate, Suspiria, while the others continued on their journey. The Caretaker created a series of underground caverns in which the Ocampa could live, and supplied them with sufficient energy to do so. However, the Caretaker did not realize that his actions were also rendering the Ocampa utterly dependent on his assistance. Suspiria did realize this, and left her mate, taking a small group of Ocampa with her. (VOY: "Cold Fire")
Some time before early 2371, the Caretaker realized that he was dying, and began an exhaustive search to try to find a compatible biological lifeform with whom he could reproduce. He assumed that only another being like himself would be capable of taking over the incredible burden of caring for the Ocampa. To further his search, he abducted dozens of starships from all across the Milky Way Galaxy, using his array conducting experiments upon the ships' crews to determine whether they were compatible. However, the search did not go well. A few of his subjects showed promise, and he sent these to live with the Ocampa for a time, but all of the subjects eventually died.
Among the ships he abducted in 2370 were a Cardassian Galor-class warship and the Dreadnought missile, and in early 2371, the Federation starship USS Equinox. (VOY: "Dreadnought", "The Voyager Conspiracy", "Equinox")
Around stardate 48300, the Caretaker abducted a Maquis raider, the Val Jean, from the Badlands in the Alpha Quadrant and transported it over 70,000 light years away to his array. He also abducted the USS Voyager in the same way. These two ships were the last vessels he would transport to the Delta Quadrant.
Shortly after abducting Voyager, and recognizing that his death was imminent, the Caretaker began sending an increased store of energy to the Ocampa reserves, so that they would have enough to last up to five years after his death. However, when confronted by Captain Kathryn Janeway of Voyager, he began to realize that caring for them was not helping them, but making them dependent on him. Captain Janeway made him realize that the "children" (as he considered the Ocampa) had to eventually grow up. To help the Ocampa along, he sent them a record of his knowledge that would hopefully allow them to develop self-sufficiency.
A short time later, before he could send Voyager or the Maquis ship back, the Caretaker died, leaving the two ships stranded over seventy thousand light years from home. Before he did so, he requested to Captain Janeway that the Kazon should not be allowed to use his array, forcing the Starfleet captain to destroy it. (VOY: "Caretaker")