Difference between copyediting and proofreading
Overview
Copyediting is a process that takes more time, is more intensive and of a broader nature than proofreading. Just to clarify and for reference copyediting includes work such as reviewing and refining the text for consistent style, voice/tone and proper grammar, ensuring proper word usage, fixing awkward phrasing, possibly pointing out problems that may require re-write, and also correcting spelling and punctuation in the process. Although the spelling and punctuation will naturally be part of the copy editing phase, it's not the last chance to fix these things.
Proofreading is the last stage before a work takes its final form, whether print or digital. At that point, all of the work listed above is finished, and it would likely be too late to get into a deeper level of change at the proofreading stage.
Proofreaders are also examining the work in its final, designed format, and so are responsible for typographical errors, the way that lines of text break and flow, etc. Proofreaders are not typically responsible for suggesting or making any kind of content changes.
For reference, I've added links here so you can have a fast reminder if the question comes up again. Also, you can use a little mnemonic trick to remember: copyediting is mainly for content, proofreading is the last stage review pre-press.