To help remember important players, they can be added as favourites. Thus you can quickly get access to their profile via the favourites menu and can write to them more easily in the message menu.
A player can also add other players as friends. In order to this the player has to be added as a favourite and then be sent a request via the 'Friend' button. This has to be confirmed by the opponent. Once it has been confirmed, the system will mark both players as friends.
The advantage of friends is:
You can communicate via the interface more quickly with friends (via messages)
A friend's online status can be made visible. There are three levels:
active
has been inactive for 2 days
has been inactive for a week