The Templars are the secret order opposing the Assassins in the Assassin's Creed series.
The fictional Templar Order seeks to impose peace and stability through control, authority, and the manipulation of powerful artifacts. The Assassins, by contrast, defend free will and oppose domination. The conflict is expressed through different organizations and historical periods, but the ideological struggle remains a central constant of the franchise.
The series connects its fictional Templars to the historical Knights Templar, a medieval military order that was dissolved in the early fourteenth century. Assassin's Creed reimagines the historical order as part of a much older and broader secret organization. This is a fictional interpretation rather than a factual account of the real Knights Templar.
The Hidden Ones are the precursor organization to the later Assassins in Assassin's Creed Origins, while the Ancients are a Templar predecessor in that game's setting. The Rooks are Jacob Frye's gang in Syndicate, not the Assassins' principal ideological enemy.