Proofreading is the third and final stage of editing for publication, and takes place after copy-editing is complete, and the document has been typeset for printing. A proofreader checks the text, line by line, for last-minute typographical errors and omissions before publication takes place. Proofreading is an important part of the editing process, as mistakes can be missed (or even created) during copy-editing and typesetting.
My role as proofreader will depend on the client's route to publication. For a client wishing to self-publish, proofreading is a must! Invariably, I would have typeset the document, so require a proof copy of the printed book to work on. For a client wishing to publish independently or submit their work to a literary agent, I do not typeset a document unless it is a specific requirement. Instead, I leave the electronic copy of the file as an A4 Word document, I tidy up the formatting a little, and then proofread the document on screen. Traditional publishers will carry out their own editorial service; some independent publishers will offer this - it's best to check.
My fee for proofreading is based on word count, and attracts an additional fee for typesetting where required.