There are few words thornier than "evangelical." It's a broad category that includes fundamentalists; it's also a reaction against fundamentalism. Possibly it's an exclusively Protestant phenomenon; to some Europeans, it's simply a synonym for "Protestant." It's a nonexclusive adjective or a group distinct from mainline Protestants, probably from Catholics, and possibly from peace church traditions.
David Bebbington and Mark Noll have characterized evangelicals by their emphases on personal conversion, the authority of scripture, activism, and substitutionary atonement. While this doesn't clear up every ambiguity, it is far more useful than simpler alternatives that confuse more than enlighten.
Mark Pinsky's A Jew Among the Evangelicals will enlighten many secular readers. Pinsky, a religion reporter for the Orlando Sentinel, demonstrates persuasively that evangelicals are neither particularly out of step with the larger culture nor tightly in step with each other. This is a recently popular theme: In 2006, you couldn't pick up a newspaper without seeing a divisions-in-the-born-again-ranks report, a profile of a moderate pastor with big ideas and a bigger e-mail list, or a headline punning on "The Changing Climate of Evangelicalism." But Pinsky's book stands alone as a fair and in-depth study by a credible outsider.