Earth-616 is the 'official' designation of the particular parallel Earth on which the majority of Marvel Comics characters live, including (obviously) most of those created by Marvel UK. It is, essentially, the mainstream Marvel Universe. All of Marvel's various alternate Earths have numerical designations. Other notable ones include Earth-238 (the 'Crooked World' from the Captain Britain stories by Dave Thorpe and Alan Moore), Earth 33 1/3 (the cartoon world of characters such as the Fairly Amazing Spider-Hound), Earth-5556 (the Earth inhabited by characters from Marvel UK's licensed Doctor Who comics, such as Sharon Davies and Gus Goodman), Earth-5555 (home of other Marvel UK characters including Spratt, the Sleeze Brothers, Death's Head and Dragon's Claws) and Earth-712 (the reality of the Squadron Supreme).
Prominent characters existing on Earth-616 include Spider-Man, Captain Britain, Psylocke, Captain America, Thor, the Hulk, Black Knight, Motormouth, Killpower, Dark Angel, Digitek, Union Jack, Ant Man, Night Raven, Nick Fury, the Knights of Pendragon, the Fantastic Four, Excalibur, Timesmasher, Rom, the Avengers and the X-Men.
It is satisfying to note that the designation "Earth-616", which is now used throughout the Marvel universe and its associated fandom, was actually coined in a Marvel UK title. Dave Thorpe apparently chose to use it in his Captain Britain series in Marvel Super-Heroes Monthly as a kind of joke, since 616 is one of the numbers associated with the Devil of Christian mythology, and Thorpe disliked superheroes.
See also