Evangelicals believe that the essence of Christianity is in the Bible, in particular, Christ's death. They believe in proselytizing aggressively as part of actually being a Christian, and in being 'born again' to be saved. It's very similar to aggressive forms of Islam that interpret certain tenants to mean that conversion of others to the faith is part of practicing it fully.
EDIT: I should say that they are always Protestants (Methodist, Lutheran, Baptist, etc.) and that they are usually what people mean when they talk about "Bible Bashers" or the "Bible Belt" in the US's South and Southern Midwest. They are are huge part of the USA's population and have become politically very powerful, in large part because of the conservative coalition's (i.e. Confederate holdouts) strategy to directly associate Christian values with regressive politics and state's rights after the New Deal. Those folks eventually became the contemporary GOP.