Its the game that came out before PSU. It was first on dreamcast and then moved to gamecube, xbox and PC. Many people think it was alot more fun than PSU, mainly cause of PSU's corny story. I think it has a lot nicer sci-fi atmosphere and although there isnt really any story in the traditional sense, there is sort of a mysterious implied story that you discover as you play through the quests. There are also alot of other small things they changed in PSU that fans really hate. Like in PSO, magic is seperate from weapons (ex. you can cast any spell reguardless of what weapon your holding), all attacks are timed(no senseless button mashing), and more interesting bosses. Argh... You really have to try it to understand the difference.
Heres the offical site for blue burst (the last version of PSO for PC) its got pictures and stuff:
http://www.psobb.com/gameinfo/index.phpThey have since shut down the offical servers but there are private servers running and they dont charge a subscription fee or anything. Also, the most recent version of PSO was free to download off of the offical website but only has online mode which as I said before will only work on the private servers.