The Marvel Universe is the shared fictional universe where most comic book titles published by Marvel Comics take place, including those featuring Marvel's most familiar characters, such as Spider-Man, the Hulk, the X-Men and Captain America. It has been covered by a number of different role-playing games over the years.
The Marvel Universe is further depicted as existing within a "multiverse" consisting of thousands of separate universes, all of which are the creations of Marvel Comics and all of which are, in a sense, "Marvel universes". In this context, "Marvel Universe" is taken to refer to the mainstream Marvel continuity, which is known as Earth-616.
Source: Wikipedia, "Marvel Universe", available under the CC-BY-SA License.