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 if you are a citizen from a EU country, the B permit category "EC" that you will receive gives you almost the same benefits as the C permit. The only extra right you will get when you obtain the C permit after 5 years is the right to vote on local issues in some cities and that of buying firearms.