The early Volkswagen Beetles (the air-cooled, rear engine'd ones) started off with a 6 volt system, the same as some other classic cars and motorbikes.
Obviously there are the 12 volt systems used in most cars and 24 volt used in trucks. The truck systems typically use multiple 12 volt batteries wired in series to provide 24 volts. This is certainly the configuration at my old MAN 7.5 tonne truck used.
It's also worthy of mention, although not in your original question, that there are two polarity of systems used in cars. Older classic cars can be "Positive Earth" where the +ve terminal of the battery is wired to the chassis and electrical systems are "wired" back to the -ve terminal via the wiring loom. Modern vehicles tend to run "Negative Earth" where the -ve terminal is wired to the chassis and the electrical system, via the wiring loom and fuse box, are attached to the +ve terminal of the battery.
You can not mix 6, 12 and 24 volt components but you typically will find that most modern trucks that run 24 volt will provide an accessory socket for 12 volt somewhere on the dashboard.
If you are asking about converting a 12volt vehicle to 24volt, this is a very involved process as not only are all of the bulbs rated at the appropriate voltage, but all of the switches, the loom itself and systems such as the ECU are all designed to be run at 12volt.
It may help if you included the reason why your are asking the question.