A C permit is a permanent residence permit. It is valid for as long as you remain in Switzerland. It has almost no restrictions and enables you to do many things not possible with B permits such as buying real estate without restrictions, moving from one canton to another, buying weapons, becoming self-employed, etc...

Depending on your citizenship, you can obtain a C permit after 5 or 10 years of residence with a B permit. Please note that European citizens now become from the very start a EU B permit that gives similar rights to a C permit.