Function
The Wii system will read and run any Wii game disk that is inserted into the system, regardless if that disk has been used on a different Wii system.
Saved Games
The Nintendo Wii saves game data to hard drive space in the Wii system itself and not on the game disk; this means that saved game progress will not be transferred to a different Wii system when using the disk to play a game on a different system.
Transferring Data
The Nintendo Wii uses SD memory cards to save and transfer data between Wii consoles; saved game data may be able to be saved to an SD card and then transferred separately to a new Wii system.
Considerations
Certain games may have saved data that cannot be copied to a memory card and transferred to another Wii system; for these games, achievements and unlockable features will have to be achieved again on the second Wii console.
Drawbacks
Popular first party Wii games (those produced by Nintendo like Smash Bros Brawl and Mario Kart Wii) often have saved data that cannot be transferred; this can be annoying since it can take many hours to unlock everything on a different Wii system.