Religious fundamentalism, or just "fundamentalism", refers to any kind of reactionary religious sect which focuses heavily on doctrine, often emphasizing violent retribution (immediate or delayed) for those who violate doctrine and promising post-mortem rewards for those who obey it in spite of personal moral objection or uncertainty.
The original usage of the term was in reference to Protestant fundamentalism, "a movement in North American Protestantism that arose in the early part of the 20th century in reaction to modernism" [W] and which emphasized that the Bible is literally true both as a moral guide and as a historical record, but it applies to any religious sect in which doctrine overrides objective evidence (i.e. religious belief-tribes).
Because of their belief in divine reward and retribution, and their separation from any objective means of evaluating the morality of an action, fundamentalists tend to make particularly good subjects for fearmongery. This is almost certainly the main reason why fundamentalism is so popular: it creates a power-resource that certain types of personality (generally, powermongers) can exploit for their own gain.
The word "fundamentalism" is sometimes used to indicate a relatively conservative position within a body of thought on any subject, not necessarily religious or philosophical in nature, e.g. "I'm a bit of a fundamentalist about quantum physics; the 'many worlds' interpretation just seems too far-fetched to me."