A person who takes their religion so literally and to such extremes that they contradict the very basis of their faith. They typically believe in a literal, verbatim interpretation of their scripture. They also have ridiculous, childish defenses to intelligent criticism of their beliefs that border on insanity. The level of hypocrisy and stupidity most of these people exhibit is truly profound.
Prime examples of fundamentalists are the geniuses who call themselves Christians and march around with signs that say "GOD HATES FAGS," seriously suggest that the earth is 6,000 years old when an overwhelming amount of scientific evidence proves otherwise, or tell women that they are filthy when they are menstruating (because it's in the Bible, you know).
Fundamentalists in general give religion a bad name. By definiition, it is impossible for any religion or belief structure that is centered on love, compassion, understanding, and forgiveness (most of the major religions are) to be anything but great. However, when people watch the news and see these dumbasses parading around with their "THANK GOD FOR AIDS - FAGS GO TO HELL" signs and calling themselves Christians, it tends to leave a bad taste in the mouth.
See also asshat
Jerry Falwell blamed the 9/11 attacks on the wrath of God, which he claims was incited by gays, lesbians, and pornography. He's a fundamentalist.