Can someone explain to me what we mean by "evangelical Christian" that distinguishes them from regular Christians?
"Evangelicalism" is a wide umbrella that has seen its meaning change over the years. Originally, it simply referred to a particular branch of actively-proselytizing and revivalist Christianity.
That changed in the early-20th century with the rise of fundamentalism and the Pentecostal movement, which were in large part complementary, and which eventually merged; resulting in what those outside the US commonly refer to as Conservative Evangelicalism. The most significant example of this branch of the religion is the Assemblies of God church founded in 1914.
The politics of the member churches at the time tended to vary, but overall tended to reject the liberal "social-gospel" of the more mainstream denominations, and espouse a conservative political platform. With the birth of the Charismatic movement, it became firmly and inextricably linked with far-right conservative politics.
The basic tenets of American Evangelicalism are those of
Fundamentalism. In a nutshell, these are Biblical literalism and inerrancy, conservatism, Dispensationalism, millenialism, and opposition to what it sees as attacks on Christianity in the sciences and social programs (it is profoundly anti-evolution and anti-abortion, as well as anti-LGBT rights; although some Evangelical branches reject the association with fundamentalism due to its profoundly anti-intellectual stands, they still agree with the majority of its tenets).
In recent decades, this has merged with the "Word of Faith" aka "Prosperity Gospel"; resulting in even further association with far-right conservatism; and emphasis on material wealth and prosperity as an indicator of piety and favor from G-D. This last quality is a very large part of why the Evangelical movement is so smitten with Trump. He is able to claim to be Christian (he's not, he's actually a disciple of Norman Vincent Peale), and hold up his wealth as an indicator of his faith has resonated with those who hold this theological worldview.