The Others, referred to by the DHARMA Initiative as the Hostiles or the Natives, and also by the tail section survivors of Oceanic Flight 815 as Them, are a group of people living on the Island who were followers of Jacob through the intermediary of Richard Alpert. Jacob never showed himself to his people, and they took orders from a succession of leaders including Eloise Hawking, Charles Widmore, Benjamin Linus and briefly, John Locke. They were "The Heavies" and the secondary antagonists-turned anti-heroes during the run of the TV series Lost.
Jacob had long been bringing people to the Island to prove his nemesis, the Man in Black, wrong about his view of human nature. According to Jacob, it is the Man in Black's belief that all people are inherently corruptible, whereas Jacob wished to show him that people could know the difference between right and wrong without his interference. For some time, the people Jacob brought to the Island were simply killed by the Man in Black or each other, without Jacob stepping in. In 1867 however, Richard Alpert arrived to the Island on the Black Rock and, in exchange for the gift of agelessness, agreed to act as an intermediary between Jacob and the people he would bring to the Island. This was essentially the beginning of the Others.
According to Richard, only one leader of the Others is appointed at a time, and this person must undergo numerous tests to get the position. According to Benjamin Linus, the leader still has to answer to Jacob. Even while he was the leader of the Others, Ben never met Jacob during his time in that role. Aside from Ben, who finally encountered Jacob in "The Incident, Part 2", the only Other definitively established as having firsthand communication with Jacob is Richard Alpert. A spy for the Others named Ethan was sent to make a list of the 815 survivors, but was murdered by Charlie before he could complete this task.
Benjamin Linus' term as the leader came sometime after the Purge, when he ousted and banished previous leader Charles Widmore. Ben stayed in power until he moved the Island in 2004, when John Locke took his place. John's term was extremely short, however, for he soon unwillingly time-flashed throughout the Island's history in a journey that eventually sent him off-Island, ending in his demise. When "Locke" returned to the Island three years after leaving, it was actually the Man in Black in Locke's form.
Prior to Ben's term, the leadership of the Others seemed to be shared between Charles Widmore and Eloise Hawking, who had a "complicated" love life together. Before them, Richard Alpert appeared to be leading the Others in lieu of a "leader" in 1954; he did mention he had a superior that he answered to. It is unclear if he was referring to an unseen leader or simply to Jacob himself.
The earliest time frame that shows the Others on the Island is in 1954, but they have been on the Island long enough to be referred to as "indigenous". They also speak various languages with each other, including Latin and Russian, to keep from being understood by foreigners. They actively recruit people from the outside world to join them on the Island (Dr. Juliet Burke, for example), and, by 2004, very few of their members had been born on the Island due the deaths of their pregnant women from an as-yet uncured medical affliction that plagued the Others some time after 1977.
They appear to have considerable wealth and power in the outside world, evidenced by the reach of their front company Mittelos Bioscience. The Others are in possession of the secret knowledge of the bearing by which the Island may be left, and also know how to locate it again to return (though the DHARMA Initiative later discovered reliable ways of getting to and from the Island). After the Purge, they gained the use of the Galaga submarine from the DHARMA Initiative, allowing them to clandestinely come and go from the Island at will.
The Others use rings of black ash to keep out (or possibly in) the smoke monster. The Smoke Monster can cross the perimeter of ash only when its continuity has been disturbed.