Since this is kinda a sermon on my own personal opinion on the matter, and I'm going to be a little preachy, I felt like putting it here instead of Religion and Philosphy because... well... stuff.
What is a fundie? Is it a fundamentalist? Well, yes and no.
Being a fundamentalist can be boiled down to believing in the fundamentals of an ideal, philosophy, or religion, and sticking to those fundamentals stubbornly. This isn't always a negative thing. You can be a fundamentalist and not be a fundie. Hell, I know conservative Christians who can be called fundamentalists and yet are decent people. Because they actually believe in the values that they view as fundamental, including charity.
Being a fundie, however, is when you place your ideals, your philosophies, or your religion on a pedestal to the point where you expect others to value them the same way you do. And that if anyone disagrees with you, they are wrong and need to be taught a lesson.
Fundamentalists are generally more open to discussion. They aren't going to change their opinions about their beliefs, but they will engage in a (relatively) fair discussion, and are more likely to end it with a "Let's agree to disagree." A fundie will declare themselves the winner 5 seconds in and insist that any argumentation against them is other people being sore losers.
And so forth. Fundamentalism, like socialism, is not an inherently negative thing. It's not an inherently positive thing, either, also like socialism. And there are good things to be fundamentalist about, like charity, racial and gender equality, and so forth. Holding those values as fundamental is good. Being a fundie, however, is inherently negative. Forcing your values on other people, even if it's about world peace (or rather, ESPECIALLY if it's about world peace) is a very, very negative thing.
And that's Brother Silveresti's sermon of the daymonthyear.