Proofreading is basically a comprehensive sweep for errors, including formatting problems, spelling, grammar and inconsistencies. If your writing is already polished and simply needs a final check for occasional hiccups, then you require a Proofreader. This is obviously the fastest service and is ideal if you have a strict deadline and/or need a second eye to check for mistakes.
Only publishers and self-publishing authors need to focus heavily on proofreading to complete the editorial process. The types of errors that are addressed by proofreading are not the type of thing that would prevent an agent or publisher from acquiring your manuscript. If you know your manuscript is riddled with grammar and punctuation errors, you need copyediting before submitting to an agent of publisher.
Here at Masters of Editing a second editor will be employed to proofread to avoid what we refer to as ‘blinding’. If an editor has worked extensively on your manuscript a fresh set of eyes is always preferred to complete the process with a final sweep for errors.