First, the word evangelical means of the Gospel. Technically all Christians are evangelicals. Just like all Christians are born again in Christ. However, a group of Christian conservatives started using those words to define themselves and it caught on.
Second, as asked, your question makes no sense because evangelicals are not necessarily a denomination with their own established doctrine. Evangelicals exist across several denominations, Presbyterians included. So, I will your answer your question as to what is the difference between Evangelical Protestants and Mainline Protestants (with a focus on Presbyterianism).
The main difference between Evangelicals and Mainline Christians1 has to do with the sources for theological interpretation. The Catholic church defined two sources for theological interpretation Sacred Scripture and Sacred Tradition. The Protestant reformation introduced the idea of Sola Scriptura: scripture alone.
Sola Scriptura is a part of Calvinist theology. However, scripture is not generally interpreted literally nor is it the only valued source. For example, there are Presbyterians who will use the Nicene creed since it is based on scripture while evangelical Presbyterians usually will not because it's not written in the Bible.
Evangelical Presbyterians carry sola scriptura much further. Their literal view of the Bible leads them to teach creation only, ordain men only, and follow other evangelical viewpoints. There are evangelical Presbyterians in all Presbyterian organizations, evidenced by the recent turmoil in the PCUSA over homosexuality. But there are at least two evangelical Presbyterian groups: the PCA and the EPC.
1I'm using this to mean the central or moderate Christian view not necessarily the correct or most popular view.