Best way to store data for a live/online game

Hi guys,

I have been searching around and can't find a specific answer for this.

If you had an online game that was hosted on a server, how would you store the data of the game.

So if you had a game where many people could join, where would you store each individual players health, levels/stats, items etc etc.

Also where would you store data about the npc's in the game, npc's health etc etc.

Pretty much if you have a live game, how do you store ALL the data so that it can be altered and updated continuously as players play the game.

I hope this made sense,

Thanks.