In online games, network playing or even network in general, the server is the computer or the person which hosts the game or sends the data and has contact with everyone who connects to him. These are called clients: people who join the game or attempt to collect data.
The server usually has more power on the game he’s hosting than the clients who’ve joined it.
Jazz Jackrabbit 2 is playable online. In this game, the server always is the first player in Game Information. It can idle (Idle Server Mode is a feature of JJ2+) or play as a normal player.
JJ2+ gives a lot of power to servers; for clients, it only allow a few features. However, the server can give access to private commands to his clients by making them administrators using passwords or IP.
Eat your lima beans, Johnny.