To drive on the Swiss Motorway system you need a Vignette (see photo below). A Vignette is a sticker that goes on a vehicle's windshield. Without it there is the risk of a fine should you drive on a Swiss Motorway.
The cost to buy a Vignette is 40 Swiss Francs. A Vignette can be purchased at Swiss post offices, petrol stations, border crossings...
Note: If you hire a car in the French sector at the Geneva Airport chances are you will not get a car with a Vignette. This can be a handicap if you plan to travel in Switzerland. As you will need to buy one.
Click here to purchase a Swiss Vignette.