author of Religion and the Culture Wars
The easiest way to explain the differences between evangelicals and mainline Protestants is to start with evangelicals, because evangelicals have a clearer set of beliefs that distinguish them than mainline Protestants do.
The term evangelical comes from the word "evangel" which is a word form in Greek from the New Testament that refers to the good news of Jesus Christ -- that Jesus came to save humanity -- and evangelicals have a particular take on the good news. That makes them distinctive from other Christians. It could be summarized, I think, with four cardinal beliefs that evangelicals tend to hold, at least officially.
One belief is that the Bible is inerrant. It was without error in all of its claims about the nature of the world and the nature of God. A second belief is that the only way to salvation is through belief in Jesus Christ. A third belief, and one that is most well known, is the idea that individuals must accept salvation for themselves. They must become converted. Sometimes that's referred to as a born-again experience, sometimes a little different language. Then the fourth cardinal belief of evangelicals is the need to proselytize, or in their case, to spread the evangel, to evangelize.
Now different members of the evangelical community have slightly different takes on those four cardinal beliefs. But what distinguishes the evangelicals from other Protestants and other Christians is these four central beliefs that set them apart.
Mainline Protestants have a different perspective. They have a more modernist theology. So, for instance, they would read the Bible, not as the inerrant word of God, but as a historical document, which has God's word in it and a lot of very important truths, but that needs to be interpreted in every age by individuals of that time and that place.
Mainline Protestants tend to also believe that Jesus is the way to salvation. But many mainline Protestants would believe that perhaps there are other ways to salvation as well. People in other religious traditions, even outside of Christianity, may have access to God's grace and to salvation as well, on their own terms, and through their own means.
Mainline Protestants are much less concerned with personal conversion. Although they do talk about spiritual transformation, they'll often discuss a spiritual journey from one's youth to old age, leading on into eternity. So there is a sense of transformation, but there isn't that emphasis on conversion -- on that one moment or series of moments in which one's life is dramatically changed.
Finally, mainline Protestants are somewhat less concerned with proselytizing than evangelicals. Certainly proselytizing is something they believe in. They believe in sharing their beliefs with others, but not for the purposes of conversion necessarily. The idea of spreading the word in the mainline tradition is much broader than simply preaching the good news. It also involves economic development. It involves personal assistance, charity, a whole number of other activities.
But on many points, evangelicals and mainliners are sometimes hard to tell apart, because there are people in the evangelical tradition who are somewhat more modernist and tend towards the mainline. We often refer to them as liberal evangelicals. But then there are also people in the mainline churches who have a more traditional, or conservative perspective. They're sometimes referred to as evangelical mainline Protestants.
So this is a little bit confusing here, because the two communities are not as completely distinct as some might argue. But there are clear distinctions at the core of each tradition, which allows us to recognize them as different approaches to Protestantism.