How do Sat nav systems determine the route to take?

[brookshuang]

I understand that there are 2 main players in the sat nav mapping market, Navteq and Tele atlas. Are they also responsible for determining which is the fastest route, shortest route etc. I have tried a number of sat navs and some routes are much more preferable to others. If I found out the drivers I could look for this when buying a new sat nav. Cheers