Proofreading traditionally means reading a proof copy of a text in order to detect and correct any errors. Modern proofreading often requires reading copy at earlier stages as well.
Proofreading in Printing and Publishing
A proof copy is a version of a manuscript that has been typeset after copyediting. 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 style manuals, by house style, or, more broadly, by the international standard ISO-5776, or, 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 (often known as a revise).
The term proofreading is sometimes used incorrectly to refer to copy-editing. This is a separate activity, although there is some overlap between the two. Proofreading consists of reviewing any text, either hard copy (on paper) or electronic copy (on a computer) and checking for typos and formatting errors. This may be done either against an original document or "blind" (without checking against any other source). Many modern proofreaders are also required to take on some light copy-editing duties, such as checking for grammar and consistency issues.
Proofreading in Biology
The term proofreading is used to refer to the error-correcting processes involved in DNA replication. In bacteria, all three DNA polymerases (I, II, and III) have the ability to proofread, using 3'->5' exonuclease activity. In eukaryotes only the polymerases that deal with the elongation (γ, δ and ε) have proofreading ability (3'->5' exonuclease activity).