In some sense, there's no difference between the two. The word "evangelism" derives from the Greek word "evangelia," which means "good news." So evangelical Christians are really "Good News" Christians. Broadly speaking, then, evangelicals are just those Christians who have heard news of God's work in the world from and through Jesus of Nazareth. Indeed, in other parts of the world - Germany in particular - "evangelical" can be just another word for Protestant. And really, Roman Catholic and Orthodox Christians are "evangelical" too: they have heard the good news of Jesus Christ and they form their lives around it. So in some ways, all Christians are evangelical, since all are living the good news of Jesus Christ.
Of course, evangelicalism is something else, too. Just like Roman Catholics use the word catholic (or "universal) exclusively to identify themselves and just like Orthodox Christians use the word orthodox ("right believing") exclusively, certain Protestant Christians have appropriated the word evangelical to identify their own interests and ideas. In general, evangelicals place emphasis upon a personal relationship with Jesus, attribute a great deal of authority to the Bible, and regard the spreading of the Christian message as central to the life of faith. Unlike Roman Catholics or Orthodox Christians, however, these ideas do not associate with single denominations. Christians with these commitments live and work both within each of the mainline denominations as well as outside the purview of mainline Protestantism in non-denominational churches. So at the very least, there is great and indefinable overlap between mainline and evangelical Protestantism.