People think that proofreading and editing mean the same thing. This it entirely not true! The truth of the matter is that there is a very big difference between the two terms. It is important to be aware of the difference, so that you can tell a proofreader exactly what is required when you upload your manuscript.
Editing involves both adjusting the English, as well as checking the actual content of the work. The content is changed to better express your ideas. Many factors are included in this process like continuity, sentence structure, repetition and clarity. Continuity involves ensuring that the ideas expressed are logically correct and there are no mistakes involved. An example is the case where you mention that variable X is 10, and then later on you mention it is 12. Sentence structure involves checking whether the sentences form correct paragraphs and are written in a structure that is correct. Run-on sentences are also considered. Repetition is checking for any ideas that have been repeated in different ways. In academic work, it is not recommended, and frowned upon, to rewrite the exact same ideas in different ways. Clarity is ensuring the ideas are clear, and do not sound confusing to the reader. Another important point to check is the flow of ideas, and whether the content flows neatly, changing from idea to idea. Generally every paragraph expresses an idea, and the next is linked to the previous. A paragraph shouldn’t abruptly talk about an entirely different thing. This will leave your readers lost as to where they are in the reading process. Consistency is also considered. Sometimes there are several different terms to use to refer to a concept, or several different ways to spell a certain word. The important thing is to spell it correctly, and keep consistent with the choice you have made. Use the same terms throughout the text. For example, if you put certain foreign terms in italics, then every mention of that term should be in italics form. Putting some of the terms in italics, and leaving some as is, is an example of inconsistency. Editing is generally repeated many times during the writing process, and involves moving things around.
Proofreading, on the other hand, is generally a final round of adjusting language mistakes in the text, prior to printing the manuscript. This process involves checking for spelling, punctuation, grammar, and typos. It is a prerequisite that must be performed before print. Authors can be surprised as to how much mistakes can be spotted after they claim the text is perfect. Although a proofreader would comment on any logical errors in the meanings and concepts expressed, this is not the job of a proofreader. Structure, coherency and clarity may also be considered by a proofreader, although this job belongs to an editor.
If you are still in the midst of writing your manuscript, and need to check the English errors as well as any logical errors in the content, then you need an editor. However, if you are done with the writing process, and need someone to check the text for language mistakes, then hire a proofreader.