In an article from the Content Spotlight, Gary Kebbel, director of AOL News, offered one of the better definitions of an online editor's job I've yet read:

They select the story, including whether to use an AP, Reuters, or Bloomberg version, select and process the photo and/or the graphic, select accompanying links, and decide what to retire from the page in order to publish the link to this story. They also select and process the video and audio, write the headlines and captions, and publish all the relevant pages. They work very much on their own, and have the huge responsibility of making sure that hundreds of thousands of simultaneous users at any given second don't see an error.