Phantasy Star Online (PSO) is an MMOG originally released for the Sega Dreamcast in 2000. A bugfix/upgrade edition was released the following year, entitled Phantasy Star Online Ver. 2. This version was also later ported to Windows but was only released in Asia. Phantasy Star Online Episode I & II was later released for Nintendo GameCube and Xbox. Phantasy Star Online Episode III was later released for Nintendo GameCube with a changed gameplay formula, in the form of a card game. In turn, the Xbox's Phantasy Star Online Episode I & II was ported to Windows with the added Episode IV, NPCs, quests, team system and server-side saving system, as Phantasy Star Online: Blue Burst.
The Phantasy Star Online titles are a sub-series of Sega's Phantasy Star of games that began in 1987. The PSO games themselves are simple RPGs where the player slays monsters, levels up, buys new equipment, etc. The online Phantasy Stars differ from the previous games in the series by offering a real-time approach to combat rather than a turn-based one, and seamlessly integrating this with the exploration/plot development aspects of the game.