Server
Definition
A server is a computer on the internet used in playing the game. A server determines the actions of each NPC; it keeps track of where each character is, what they look like and what they are doing; and it remembers your character's name, level, costumes and everything else while you are logged off.
The more players using a server, the more work it has to do. If too many people use the same server at the same time it has a hard time keeping up with all their actions and things might slow down or get jerky on the screen. Servers are selected by using different manifest files in the Launcher.
During Live there were no manifest files and the server was chosen after login. What used to be called servers then are now referred to as Shards now. Most servers only run one shard.