They are the sworn enemy of the Assassin Order, who, despite their name, are more or less the peace keepers in this ongoing war. The Templars truly long for world domination using ancient artifacts believed (and in the story eventually proven) to hold mystical powers.
The Templars are based on, and infact are the Knights Templar, a Christian band of knights from the 12th century. As well as their rivals the Assassin Order being based on the real life band of assassins known as the Hashashin (Hashashin believed to be the word "assassin" came from). In all reality these 2 groups did have their own bit of history together but, were never arch rivals nor was one or the other the bad guy. The Knights Templar were most likely the more noble of the groups, but that's fairly debatable.
templar- you have choosen... wisely
* cool ass last crusade music"
That level 6 Paladin templarred the orc with his +1 bastard sword, dealing it 7 damage!