In Switzerland both gasoline and diesel are used. You will definitely be informed of which one to use when renting a car. See the picture below for a selection of cars from Avis as example. 'Mixed' doesn't mean that you can use either, it just means that in this class they have both gasoline and diesel cars.
When refuelling, you will have the option of gasoline (Bleifrei or sans-plomb, which means unleaded, sometimes followed by the number 95) and diesel. The diesel pump is always black and sometimes has a cover to make sure you don't accidentally use the wrong fuel. Also, there are two types of diesel pumps, one for trucks style cars and one for diesel cars. The one for cars has a different nozzle size.
There are other types of fuel sometimes with names such as Super 98, this just indicates a type of premium fuel and you are unlikely to need this unless specified by the rental company.
As for the process of tanking, there's two options:
- At a manned station usually with a little shop, you fill up your car first, then you go to the shop, indicate the number of your pump and pay however much you pumped.
- At unmanned stations and often at night when the shop is closed, you enter your credit card details at a machine at the pump. Again, there's no need to indicate how much petrol you want, you just pump as much as you need and it will be deducted from your card.