The Earth Campaign is a first edition Advanced Dungeons & Dragons game melieu. Most of the action takes place on three worlds: Earth (my own campaign setting), Oerth (TSR's Greyhawk setting) and Middle Earth (ICE's game setting, though played with AD&D rules). There are many smaller worlds as well that the parties visit occasionally.
Time-wise, the game is set some 80,000 years in the future. Between now and then, humankind developed a great space-faring civilisation which spanned not only this universe but adjacent universes as well. All of the other races which populate Earth, Oerth and Middle Earth were contacted during this time. Like all great empires/civilisations, though, it eventually declined and collapsed, leaving mankind, their friends and their enemies scattered across millions of worlds. Technology declined, until only primative agrarian societies existed on the three main worlds. However, in the meantime the powers of magic and psionics had been discovered, and these are growing to fill the gap left by the loss of technological prowess.