The server-client relationship

When talking about multiplayer games, people will talk about the server. "I'm going to join the server." But what does that mean exactly? How do the different computers involved in the game connect to one another?

One state to bind us all

In the world of Unreal, the server is king. It holds the one true game state, while the clients connected to it simulate it as best they can. The clients run the same code and predict where objects in the game world are by using their last known location and velocity, until the server updates the client with their actual positions. If you've ever experienced heavy lag while playing online and had the characters seemingly teleport around, then you know what happens when this prediction model goes too far out of sync.

In the client-server model each player (client) connects to the server and is unaware of other clients connected to that server. That might not make sense at first, but if you think about it each player in the game...