Editorial proofreading is what most people outside of the publishing industry think of simply as proofreading. The document has already been copy edited and formatted. Proofreading is the very last quality control step before printing the document or making it public. I will:

• Correct minor mechanical errors that were missed during copy editing (such as spelling or capitalization mistakes) 
• Correct egregious grammatical errors
• Correct deviations from the editorial style
• Note minor design/formatting flaws that may have been introduced inadvertently when the document was imported to its final medium (such as incorrect pagination, extra spaces, misplaced page breaks) 

Note: It is best practice to have an editor who has not previously worked on the project do the proofreading. A completely fresh pair of eyes is critical to catching minor errors and design flaws.