Short answers, pulled from the story.
The word Sith first appeared in a 1974 rough cut of the Star Wars script. This early draft used the term to describe the villain Darth Vader before the official title Dark Lord of the Sith appeared in the 1976 novelization.
Linguists trace the root of Sith to two distinct sources: Edgar Rice Burroughs' 1914 science fiction novel The Warlord of Mars and Scottish Gaelic folklore. In Burroughs' book, Sith refers to a species of beetle, while the Gaelic sith describes a type of spirit or fairy.
Darth Bane established the Rule of Two which stated there must be only two Sith Lords at any time: one master to embody power and one apprentice to crave it. This law guaranteed the conspiracy remained secret for a thousand years by ensuring each apprentice eventually challenged and murdered his or her master.
Sith were formally introduced on-screen with the release of Episode I: The Phantom Menace in 1999 as a shadowy martial order manipulating political factions into galactic civil war. They were first mentioned in a scene ultimately cut from the original 1977 film Episode IV: A New Hope.
Darth Vader became Sidious's third apprentice after turning to the dark side following Padmé Amidala's death during childbirth. He was encased in black armor with extensive cybernetics after losing his organic arm and legs in a duel with Kenobi on Mustafar.