Gollum was once a hobbit of the Stoor strain, orinally called Sméagol. He dwelt on the banks of the Anduin River near the Gladden Fields. Sméagol went fishing with his friend Déagol one day, and Déagol, according to the story, fell into the River. At the bottom he found the One Ring, but thought it merely to be a worthless, yet pretty, trinket. When he got back on dry land Sméagol demanded that Déagol give him the Ring, but Déagol refused. It was Sméagol's birthday, so he tried to use that as an excuse to get the Ring, though it was unsuccessful. This is why he later referred to the Ring as "my birthday present." Frustrated, he killed Déagol in one way or another and took the Ring. From that point on he used it to steal things from his fellow hobbits until they discovered the thief's identity, at which point they cast him out. He received the name Gollum because of the disgusting slurping sound he made in the back of his throat.
Gollum soon became angry at the sun, for it burned his back one day, and thus he sneaked into a cave at the roots of the Misty Mountains, where he dwelt with his Ring for almost 500 years. Bilbo Baggins soon stumbled into the cave and found Gollum's Ring, and soon after met Gollum. Gollum challenged Bilbo to a riddle game, which Bilbo won. Gollum, in an attempt to kill Bilbo, searched for his Ring, but could not find it. Bilbo, accidently slipping on the Ring, escaped from Gollum's clutches.
Soon after Gollum left the cave in search of his Ring, which he called "my precious." He called himself the same, and it is possible that he got the Ring confused with himself. He searched far and wide, but could not find his Ring until one day he came upon Mordor, and there he was captured and brought before Sauron. It was possible that he was tortured, but in one way or another he gave away the name of Baggins, and thus Sauron set forth the Ringwraiths. Gollum was released from Mordor to find Baggins, but was captured by Aragorn and interrogated by Gandalf, at which point he learned that Bilbo's Ring was almost definitely the One. Gandalf put Gollum in the jails of the Mirkwood Elves, which is probably why he had such a hatred for Elves. An Orc band one day raided the Elven King's home whilst Gollum was outside climbing a tree that the Elves allowed him to climb, and after the raid was through and the Orcs escaped, Gollum was gone.
He persued the Company of the Ring through Moria and then down the Great River until the breaking of the Fellowship. At that point he began to follow Frodo and Sam, and eventually they caught him and made him show them a way into Mordor. He apparently truly wanted to help, for without the Ring he was becoming more hobbit-like once again, but in the end he betrayed the hobbits and told Shelob of their whereabouts when they passed through Cirith Ungol. They escaped, but later, in the Chamber of Fire, Gollum attacked one last time, and took the Ring and Frodo's finger, falling with both into the Crack of Doom. He died with this final motion and the Ring was destroyed with him.
By Alan Lee.