www.m-w.com decribes a copyeditor as one who prepares copy for the typesetter or one who edits and headlines newspaper copy.
A proofreader is defined as someone who reads and marks corrections in typeset text.
So clearly, a copyeditor is the one who edits copy before the proofreader. After the copyeditor is done with his or her job, the next stage involves proofreading. This is at the typesetting stage and often the proofreader has to check every single word so that no “typos” are found.
Aside from this basic difference, here’s a list of tasks a copyeditor does: http://www.sfep.org.uk/pub/faqs/fedit.asp
This is a list of tasks a proofreader does: http://www.sfep.org.uk/pub/faqs/fproof.asp