These are two words thrown around a lot in Biblical studies. Fundamentalism is also a word thrown around a lot in the culture at large. Westboro Baptist Church is seen as a fundamentalist church; Al-Qaeda is seen as a fundamentalist Muslim group. In these cases, fundamentalist basically means “so conservative that they’re bad.” The underlying idea seems to be that to really, truly believe the tenets of Christianity or Islam is dangerous.
John Yoder, in his essay “A Theological Critique of Violence,” defines the terms this way.
I define fundamentalism as that form of theological culture that assumes there are no hermeneutical problems, since what I take to mean is what it has to mean. Foundationalism… makes a similar but opposite mistake. It assumes that since there are hermeneutical problems, we should and can resolve them before entering into the substance of the debate by making a ruling on how terms must be used.
In Yoder’s view, it would seem that fundamentalism isn’t a problem of how strongly or how conservatively we believe whatever we believe, but how adroit we are at engaging those who see the same things differently. What is important is not holding our own beliefs at arms’ length, but learning how to negotiate disagreement. As such, definitions become vitally important.