Part of the job of a good proofreader is to not only find the errors, but also having the ability to find the write words (yes, I did that on purpose) to get the author's message across to the reader in a clear and concise manner.
Proofreading itself involves pretty much the obvious: misspelled words, incorrect punctuation and grammatical errors. One of the most common "problems" facing a writer is the use of there, their and they're.
You won't find the answer here, however - that's my job, and very possibly the topic for another blog post!
Copyediting is another aspect and is a bit more involved. When editing written material, a copyeditor looks for proper structure and "flow." The last thing a reader wants is to have to work at reading. The written words should be as readable as if they were being spoken aloud. Of course this goes without saying that in such case, the speaker be well-versed in the proper use of the English language as well.
So, you have basic proofreading for spelling, punctuation and grammar; copyediting for structure and proper content; and what else? Writing. In order to be of value to an author, writer or publisher, a "proofreader" should definitely possess excellent writing abilities. If something is written without proper format or composition, how could that possibly be remedied unless your editor has the ability to find the right words and use them in the correct order, so that what you are writing is understandable and reads with ease?
The importance of a good proofreader/copyeditor is obvious to the majority of people in the "world of words." In this post I hope to have conveyed that message and defined in a bit more detail the different aspects involved in the field most commonly referred to as proofreading.