In his book Understanding Fundamentalism and Evangelicalism, George M. Marsden (professor of the history of Christianity in America at the Divinity School, Duke University) uses the introduction to define his terms. He begins with a little levity, saying that "a fundamentalist is an evangelical who is angry about something." Then notes that it is basically correct (adding that Jerry Falwell uses it a quick definition to give to reporters).

He then goes on to give a more specific one:

an American fundamentalist is an evangelical who is militant in opposition to liberal theology in the churches or to changes in cultural values or mores, such as those associated with "secular humanism." In either the long or the short definitions, fundamentalists are a subtype of evangelicals and militancy is crucial to their outlook. Fundamentalists are not just religious conservatives, they are conservatives who are willing to take a stand and to fight.
One should view the term "militant" in context, rather than imagining actual aggression or desire to attack (though it is difficult not to find such things as " onward, Christian soldiers, marching as to war" suggestive). He goes on to explain that they are not denominations or religions unto themselves but "religious movements" (italics in original) within Christianity. Movements that have subgroups, debate, disagreement, and dissension within ranks as with any other movement. They are general terms with a certain amount of diversity under each heading (more so with evangelicalism).

What is known as evangelicalism and fundamentalism largely grew out of the "revival" and millenarian movements of the 19th century that found themselves becoming more and more opposed and at odds with what was a growing liberalism in theology (which in turn, grew into what is usually referred to as "mainstream" denominations today). The changes were partly due to biblical scholars critically examining the Bible, its history, biblical history, et cetera. Discoveries in science: geologists (most practicing Christians) finding no evidence of a Noachian flood and evidence for a world far older than Bishop Ussher's calculation and Charles Darwin's and Alfred Russel Wallace's theory of evolution caused further opposition to core beliefs. The denominations that became the mainstream or more liberal assimulated and/or harmonized that knowledge and the "higher criticism" of biblical scholars to a large degree. Those that remained religiously conservative moved away from that.

By the 1920s, an even more conservative wing moved more toward separatism (even from the more evangelical denominations) and took the self-designation "fundamentalist" based on a series of twelve paperback volumes titled "The Fundamentals" (published between 1910 and 1915). The books, largely a defense of so-called fundamental Christian doctrine, were highly conservative and had work in them by anti-modernists, those against the "new liberal theology," and others who among other things attacked current Biblical criticism and aspects of science (anything that suggested a different world/human history than found in Genesis). They came following a flowering of "activist evangelicals," led by men such as Dwight Moody (founder of Chicago's Moody Bible Institute), who put on Bible conferences and seminars and opened "Bible Institutes." Networks were formed, furthering the movement toward separatism from "the mainstream."

While this isn't supposed to suggest cultic or "militia-style" separatism, one should consider the number of media outlets, daycares, publications, schools and colleges (often separate from even evangelical-based ones) catering specifically and often exclusively to conservative Christianity. Many use homeschooling as a means to insure that their beliefs and values are passed on to their children. But as earlier, this is still a general thing, intended to explain and describe trends of the movement. Just looking at the way some fundamentalist and evangelical groups are willing to become involved in politics should show that there are lines that are blurred.

Unlike many fundamentalists, evangelicals are less separatist and belong to a diverse group of (Protestant) denominations (Marsden notes that the Lutheran church uses a broader definition of evangelical that is essentially the same as "Protestant). There is less tendency to be anti-modernist (though, they are still religiously conservative) among evangelicals than fundamentalists and there is less of a sense of "them against us." They are still strongly "Bible-based" Protestants who reject much of the liberal theology found in the "mainstream" churches.

One might suggest that it is more a difference of degree, rather than kind. Another thing separating both groups from each other and other Christian denominations is the degree of inclusiveness as to what constitutes being a "Christian." Fundamentalists have a very strict notion of what a Christian is, while evangelicals are less stringent. Ecumenicalism tends to be rejected by fundamentalists, while it has some (limited, with reservation) acceptance among evangelicals.

(Primary source listed above)