The main characters in Lord of the Rings are those whom the plot is centred around and who have played crucial and pivotal roles in the story.
Frodo Baggins
Frodo Baggins is a protagonist of Lord of the Rings . He is a Hobbit in the world of Middle-Earth and the son of Drogo Baggins and Primula Brandybuck, who both drowned in a boating accident. He was adopted by Bilbo Baggins, because Bilbo said, "we can celebrate out birthday-parties comfortably together." They were both born on the same day. He acquires the One Ring from his Uncle Bilbo and begins an epic journey to Mordor to destroy it. In Tolkien's timeline he lived from 2968 to 3021 of the third age of Middle-earth.
In Peter Jackson's film adaptations of Lord of the Rings, Frodo is played by Elijah Wood.
Gandalf
Gandalf was a wizard in The Hobbit and The Lord of the Rings. To the Elves and Men of Gondor he was known as Mithrandir, the Grey Pilgrim. "To the east he goes not."
He is one of the five Istari, angelic spirits sent by the Lords of the West in human form to motivate the inhabitants of Middle-earth to resist the evil of Sauron. In the West he was the Maia known as Olorin, and once served the Vala Manwe.
During the events of the series, Gandalf plays a relatively subdued but vital role, initiating the events by providing Frodo with the ring and later engaging in a political battle in an attempt to unite the forces of Middle-earth against Mordor. At the conclusion of the War of the Ring he accompanied Frodo, Bilbo, Elrond, and Galadriel on a sea voyage to the Undying Lands in the west.
In Peter Jackson's film adaptations of Lord of the Rings, Gandalf is played by Ian McKellen.
Gollum
Gollum (also known as Smeagol) was of a hobbitlike race, and a bearer of the One Ring for many years.
In his early days, Smeagol lived in a community of a type of hobbits, who unlike most were water-folk. Once on a trip, his friend Deagol found a ring in the bottom of a pool; Smeagol coveted it, and murdered Deagol. Smeagol later found he could become invisible by wearing the ring, and put this ability to evil uses such as eavesdropping and stealing. He was exiled from the community because of this, and he sought refuge under the Misty Mountains after some time, because of his developed hate for both the Sun and moon, eventually turning into the creature known as Gollum.
Gollum's ring is eventually taken by Bilbo Baggins, and after overcoming his fear of the sun he sets out to recover it. From this he is involved in many of the events in the story, first revealing under torture the location of the ring to Sauron, and later posing as a servant to Frodo in order to reclaim his ring. This final scheme is successful, albeit briefly, as Gollum takes the ring moments before slipping into a chasm of fire at Mount Doom.
In Peter Jackson's film adaptations of Lord of the Rings, Gollum is played by Andy Serkis
Sauron
Sauron is the evil Dark Lord who is the main antagonist of the Lord of the Rings series. Sauron was originally a servant of the fallen valar Morgoth. In the second age Sauron created the rings of power: nine for men, seven for the dwarves and three for the elves. Then, unknown to the bearers of these rings, he created another ring to rule them all. He used this ring to control Middle-earth.
An alliance of men and elves rose to challenge Sauron at the base of Mount Doom. Amidst the battle, Isildur's father was slain by Sauron himself, and Isildur picked up his father's sword and severed off his fingers. The One Ring was removed from Sauron's body and his body vaporized, but he was however not completely vanquished since Isildur would not destroy the One Ring when given the chance. Sauron later attempts to regain his power and his ring, but he is thwarted by the efforts of Frodo Baggins and the Fellowship of the Ring who finally manage to destroy the ring by throwing it into the fires of Mount Doom.