Sword Art Online
    The first virtual game world, and the setting of the first and second novels of the series. It consists of a giant floating castle called Aincrad, with 100 floors in it, each of which contains a slightly different medieval setting and dungeon with a boss. If the boss is killed the players can proceed to the next floor. It is turned into a death game by its creator Kayaba Akihiko in hopes of creating a true virtual reality. It implements a level based system. Thus, in cases where players with an overwhelming difference in levels face off, one side could very well be invincible compared to the other side

Alfheim Online
    The setting for the second arc of the story. It implements the first unique flight system that allows players to fly via wings. Players must choose out of nine different races, known races are: Spriggan, Sylph, Undine, Gnome, Salamander, Cait Sith, Leprechaun, Imp and Puca, each possessing their own strengths and weaknesses. ALO implements a skill based system as opposed to a level based system, in other words all experience is used to increase the skill level with only small health increases. Thus, even lower level players have a chance to win against higher level players.

Gun Gale Online
    The setting for the third arc of the story. It is a virtual game world with its main focus on guns, although melee weapons like lightsabers and knives also exist.

UnderWorld
    The setting for the fourth arc of the story. According to Kirito it is, graphically, the most realistic of all VRMMOs to date. The flow of time in UW is faster than the real world (probably a prototype of the accelerated world). Even so, it seems that even Kirito was mislead as to the actual rate of the flow of time so an accurate ratio of the flow of time is, as of yet unknown, or whether the rate changes during the Alicization Arc. However, in the "There is but one ultimate way" side story, the rate is said to be 10000 times that of the real world.

only SAO r deadly

Alfheim Online(ALO) is a new game created after the SAO incident using a new gear(Amusphere) instead of NerveGear as it have a safety mechanism, ALO is on the 3rd and 4th volume where Kirito dived there to save Asuna after getting a clue that she's there

after this arc is the big turning point for the story as it begins the birth of The Seed, a software that allows u to easily creates a new VRMMORPG thus giving birth to tons of new VRMMO game and player can even swap to other games by using existing game data and level

Gun Gale Online(GGO) is not a death game, but Kirito is asked to investigate that game as there seems to be some high level player found dead in their room and some people saw him getting shot in town before disconnected thus the rumors of Death Gun, gun that can kill someone in the real world

for Underworld i think Shinkawa already mentioned most of them