Often misdefined as "fanatic". A person can be a fanatic and a fundamentalist, or only a fundamentalist, or only a fanatic, or neither.
The action taken by a fundamentalist depends on the nature of their belief. It can be a personal action which affects no one else, or an action which affects tens of thousands of others, or something in between. Can be positive or negative.
Can apply to any religion or ideology, also of a method of or pertaining to fundamentals.
"islaamic fundamentalist" - generally stereotyped as jihaadist. Not necessarily so.
"christian fundamentalist" - generally stereotyped as bigoted and in-your-face. Again, not necessarily so.
And a correct but uncommon use:
Music teacher: "You need to learn your instrument's fundamentals before you can play solos."
Kid: "Teacher is a fundamentalist."
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.
If a woman would rather have an abortion than; have a child she will resent, cannot support, had from rape, endanger her own life, etc... she should be able to.
If a person wants to get a divorce, because their significant other is a fucking dickhead, they should be allowed.
If someone believes that committing certian acts will condemn them to a firey hell, as based on an elaborate book of myth, they should be able to.
If someone commits an act described by a book of myth to be a sin, and they do not believe the fairytales described in said book, they shouldn't have to take shit for it.