Gollem is a villainous character, always scheming to steal items of jewellery, especially rings which he thinks are "precious".  Many years ago Gollem was a River Hobbit called Smeegle until he ambushed King Eiseldorff and stole his magic ring while Eiseldorff was swimming.  Smeegle tried to use the ring to turn himself into a frog so that he could swim faster than any of the other River Hobbits. Unfortunately, the spell didn't work and he was forced to remain half frog/half hobbit for ever with the Hobbit half, Smeegle, always arguing with the frog half who calls himself "Gollem"!

Years later, still trying to steal precious jewellery, he tried to trick the mad Hobbit, Bilbo Baggins into giving him another magic ring in the middle of a riddling competition.  He didn't manage to get his slimy flippers on Bilbo's Ring so he decided to get his revenge by telling the evil Wizard Sauron that Bilbo's ring had enough magic to allow him to control the whole of Middelerth.  When Sauron heard this, he sent his "Ringwraiths" (or Ring-hunters) to find Bilbo's ring and Gollem followed them, hoping to snatch the Ring in the confusion.

When Bilbo decided to go into hiding and gave the ring to his son, Frodo, Gollem saw his chance and befriended Frodo, telling him that he could lead him to Mordar to hide the ring in the volcano of doom.  However Frodo didn't know that all the while Gollem was hoping that his pet spider, Shelbob, would be able to steal the ring from him...