Journalism is defined as "the collection and editing of news for presentation through the media." Online journalism is simply a news presentation through online media. This type of journalism comes in many forms, some of which are not published anywhere else. Blogs can be seen as one form of online journalism. Online videos might be seen as another type of journalism. Often, online journalists do not adhere to the same ethical standards as other journalists. As post 2 points out, there is little accountability, standardization, or oversight of this type of journalism. Information and journalism is present everywhere on the web in many, many different forms. The challenge for viewers of online journalism is often to determine what is fact and what is fiction.
Basically, online journalism is a category of media that is "published" on the Internet. Journalism is usually defined as the "reporting of facts," so online journalism can include almost anything (current events stories, entertainment news, sports, etc) and is visible on the web.
One difference between online journalism and print journalism, is that almost anyone can put something online. There isn't the same system of editing, cite sourcing, or accountability that exists for print media.