In Lost, Jacob was known as the island’s protector before Jack’s final sacrifice. Jacob guards the island against his brother, the Man in Black.
Jacob lives for centuries and influences the lives of candidates around the world. He brings or guides people toward the island as part of a search for someone who can replace him. His role is revealed gradually through flashbacks and encounters with characters such as Locke, Ben, and Jack.
The series distinguishes Jacob from the island itself and from Richard Alpert, who serves as his intermediary. Jacob is also not the same person as the Man in Black, although the two are brothers and their conflict drives much of the mythology.