I am asking myself for quite some time this question.
With a normal laptop you can have at least two internet connections: ethernet (wired LAN) and wifi (wireless). If you use a USB cellular modem (GPRS, 3G or LTS) you can add one more connection.
The difference between ethernet and the wireless connections is the available bandwidth. And the cellular internet connection has in most cases some kind of cost associated.
So, how does Linux decide which connection to use for the internet traffic?