From Higher Up and Further In, a discussion of copywork:
When I first heard about the concept of copywork I was not impressed. I have been a teacher in some capacity for as long as I can remember, and one thing that stood out in my mind was the fact that children despised copywork. After some careful research, however, I learned that it need not (and should not) be a tedious exercise, and the benefits sounded tremendous. So, I gave it a whirl and am now a believer in this little timeless method of writing instruction.
Its purpose is to improve the child’s handwriting, expose them to noble thoughts, good sentence structure, rich vocabulary and introduce basic punctuation and capitalization rules. Even grammar is incidentally taught but this is just an added benefit and not the purpose for copywork.
She folllows with a good list of what her children actually do. I find this helpful as most discussion I’ve seen discuss the principles but not the praxis. Or at least not as clearly.
She also links to a forum on Ambleside Online, which looks good, though quite a bit of stuff to sift through.