Many people use the general term proofreading for what editors distinguish as copy editing. Copy editing involves correcting a manuscript for grammar, spelling, punctuation, style adherence, consistency. If you require editing services and manuscript correction, please see "Editing."
For our purposes, proofreading is performed as one of the last stages of publishing, after editing has been completed and the final manuscript has been sent to the publisher for typesetting. Once the page proofs have been created, proofreading involves the careful comparison, line by line, between the manuscript copy and the page proofs, marking and correcting any typographical or formatting errors introduced during typesetting into the page proofs.
I mainly proofread onscreen, marking up the proofs with the Tools and Comments function in Adobe Acrobat. For more information, please see Editors Canada's "Definitions of Editorial Skills."