This checks for spelling, punctuation, capitalization, and other grammatical errors. It also involves looking at sentence structure, word choice, and missing words. Essentially, it looks at correcting the language of the text.
As the name implies, this involves checking the content of the document. It checks for factual errors, inconsistencies, discrepancies in plot or dialogue. Essentially, content editing evaluates the content in detail. It makes sure what is said on page 4 is consistent with page 40.