This is such a good question!
Copperplate is a general term used to describe several styles of shaded scripts (called hands) done with a pointed pen. While the differences are subtle, Roundhand, Engrosser’s Script, Engraver’s Script, Anglaise, and English Script are all frequently referred to as Copperplate.
Beginning in the late 1500’s, English Roundhand Script was written with a quill and frequently engraved onto copper plates. Thus the term Copperplate came to be. In the 1800’s, penmen emulated the engraved style with steel nibs and the terms Engraver’s Script and Engrosser’s Script were used interchangeably.
While English Roundhand was written like handwriting, Engrosser’s and Engraver’s Script were drawn using individual strokes.
I believe Ken has posted a sample of each somewhere, let me see if I can find it and will post a link.