In the case of MMO architecture here are the relevant terms:
- Shard: Encompasses all parts of a single game world
- Zone: Area of the game which can't be further broken down; the players in this area are all connected to the same node and performing actions there.
- Instance: Area of the game which is unique for a player or group of players. This is done so that players can be together and not be interrupted by others, e.g. someone killing a boss before the group gets to it and having to wait for it to respawn. These are created on demand by players and go away after the players leave.
- Node: Single machine that runs parts of a shard. This could be one zone with a high average player count, or a bunch of zones with fairly low player counts (e.g. large open areas) and maybe some instances thrown in.
Some details here can change from game to game, but most of it's pretty general.