Evangelicals stem from fundamentalists, but have diverged since the 1950s. Theologically, they hold much in common, but primarily differ in their approach to dealing with society at large. It should be noted that neither is a "denomination" but rather a reforming trend that is cross-denominational, but bound by common purpose to reform the church, stripping it of historical (and liberalizing) accretions of practice, and returning it to the same position as "the first church," centering on its devotion to Jesus Christ.
Fundamentalists sprung up during the "back to the fundamentals" movement of the early 1900s. The seminal Work was a series of essays called The Fundamentals, highlighting such luminaries as R.A. Torrey, B.B. Warfield, and E.Y. Mullins. A brief look at the titles of the essays will highlight the issues important to these reformers.
This movement was deeply affected by the Scopes Monkey Trial of 1925, and largely disengaged from society shortly thereafter. While trying to reform the church, fundamentalists eventually came to the conclusion that they must be separate from the wider culture. Evangelists like Billy Sunday preached a Gospel of being removed from the society and rejecting all its ways. This included a rejection of politics as well as an eschewment of what the culture at large did.
By the 1950s, some in the fundamentalist movement like Carl Henry - the editor of Christian Century and founder of Christianity Today, came to believe that fundamentalists must engage the culture. In 1942, he founded the National Association of Evangelicals, and the point was to be "in the world, but not of it."
With Roe v. Wade, Evangelicals began to become more involved in politics, and helped bring about the rise of the "Religious Right." In the 1980s, fundamentalist Jerry Falwell introduced an innovation to fundamentalism when he sought to bring fundamentalists also back into the political arena, albeit in a more confrontational, "culture war" sort of way.
Theology Unplugged has two podcasts (7/9/06 and 7/8/06) that do a great job of tracing the history and divergence of the two - but basically boils down to approach rather than belief. John Michael Patton (Dallas Theological Seminary) says "Evangelicals are nice fundamentalists," by which he means, they engage rather than separate from the culture.