Wikipedia There is more information available on this subject at System Link on the English Wikipedia.

Two Xbox 360s with an Ethernet cable.

System Link is a form of offline multiplayer gaming in which Xbox or Xbox 360 consoles connect to each other over a LAN. A network switch and standard straight-through Ethernet cables may be used to link multiple consoles together, or two consoles can be connected directly. Connecting two Xbox consoles to each other without a switch requires a crossover cable, while Xbox 360 consoles can use standard cables.

System Link allows for multiplayer gameplay on multiple consoles, providing an alternative to split-screen gameplay and supporting far more players in one game. Halo: Combat Evolved, through a combination of System Link and normal split-screen gameplay, allows up to 16 players to participate in a single match—four consoles, each showing four split screens. Later, post-Xbox Live games such as Halo 2 and Unreal Championship would support more than four consoles using System Link.

Keeping the tradition, the Xbox 360 also allows you to play system link. The Xbox 360 can not only use wired Ethernet to connect to a LAN, but also use a wireless adapter (such as the Xbox 360 Wireless Network Adapter in an access point-based or mesh network). To hook up 4 Xbox 360's together you need special linking adapters that you can buy quite cheap at nearby electronic stores. Additionally, some Xbox Live titles like Halo 3 can play a match with players connected over Xbox Live and on the same LAN. By allowing multiplayer games over the same LAN, System Link has led to the formation of private and public LAN Parties.

It appears more developers have been utilizing the feature in the seventh generation of video game consoles, as more 360 games are released and people started to buy second consoles, it became difficult for the average gamer to find out what games support system link.

To make it easier for players to know if a game supports System Link, a System Link 2-16 symbol has been added to the packaging of games compatible with the feature.