A proof copy is traditionally a version of a manuscript that has been typeset after copy editing, but the line between copy editing and proofreading is thin. When handwritten originals were common, it was often easier for a copy editor to review and mark up a manuscript after it had been typeset—with computer typesetting from electronic manuscripts, the distinction becomes even more blurred.

Proof typescripts often contain typographical errors introduced by mistyping (hence the word typo to refer to misplaced or incorrect characters). Traditionally, a proofreader checks the typeset copy and marks any errors using standard proof correction marks (such as those specified in the international standard ISO-5776 or more specifically for English, the British Standard BS-5261:2). The proof is then returned to the typesetter for correction, and in many cases the production of a second proof copy (or "revise").

Today, proofreading also refers to reviewing any text, hardcopy or electronic copy (on a computer), and checking for all types of errors. Many computerised word processors offer the ability to "Show Formatting Marks", displaying them as visible characters which allow proofreaders to ensure that there are no multiple spaces or stray carriage returns present in the text.