Copyright refers to the rights that automatically exist in a piece of work as soon as that work is created. The owners of those rights can transfer, assign, license, sell or otherwise hand over those rights in full or in part to second parties in the form of publishing rights, performance rights or reproduction rights, usually in return for one of those much-coveted cash-and-a-contract combos.
Copywrite is an odd hybrid of copy and writer, two words which work perfectly well on their own; or perhaps it’s a barely-truncated truncation of “copywriter”, which isn’t much better. It doesn’t appear in my dictionary but might just feature in George Orwell's 1984, in which the Ministry of Truth compounded and reduced words and phrases to neutral nonsense. Its only relationship to copyright is that the two words sound the same and no, despite what some internet posters might insist, it isn’t a legal term.
I hope that’s clear.