Most people think of a copy editor as someone who primarily looks for typos. However, in reality copy editors do a lot more than that and are indispensable to any newspaper, magazine, or other publishing enterprise.
Copy editors do proofread; however, they are not just proofreaders. Proofreaders--a rare job these days--simply scan for typographical and mechanical errors. Copy editors do this task and more.
Believe it or not, many copy editors are trained journalists--especially those that work at newspapers --and have performed significant amounts of reporting in their day. This experience comes in handy for newspaper and magazine copy editors, who have to edit reporting and occasionally do short and miscellaneous reporting of their own.
Fact checking and keeping an eye out for libel (defamatory untruths) are one of the key roles of many copy editors. Publishing incorrect facts or defamatory writing can cause irreparable damage to a publication's reputation. It's a copy editor's job to make sure this doesn't happen.
Copy editors are normally the last people to read something before it goes to publication. So if something is badly written, it's often up to the copy editor to make the writing look good, which can either mean doing some heavy editing themselves or sending the article back to the author with suggested revisions.
Copy editors are often tasked with writing headlines and captions, as well as the regular blurbs and briefs that occur in many publications. Many readers take these aspects of a publication for granted, but in truth they are some of the most important elements of a newspaper or magazine.