Wahabism is the branch of Sunni Islam which is the official religion of Saudi Arabia. It was founded by Wahab, an 18th century literalist, who some equate to a Puritan or revivalist figure. It became the official religion of Saudi Arabia. It is particularly intolerant of Shia and Sufi faiths which it considers heretical.
Madrassas funded with Saudi money in the 20th century have followed a wide range of practices and taught a wide range of doctrines, some of which are clearly associated with terrorism, and others which are wholly tolerant. Saudi funders are often accused of attempting to deliberately spread Wahabism.
Interestingly, until Osama bin Laden began using Wahabist-like rhetoric in pursuit of a renewed Islamic caliphate, there was little or no association between this particularly Saudi religious strain and any pan-Arab or pan-Muslim movement. Sunni movements originating mostly in Egypt had traditionally been associated with militant Islamic groups and provide rhetorical cover. For this reason, also until recently, most Western analysts considered Wahabism highly unlikely to spread even to other Sunni nations.