There are two main types of toilets: gravity-fed and pressure-assisted. Gravity-fed versions require as little as 10 pounds per square inch of water pressure, while pressure-assisted options use at least 25 pounds per square inch to flush.
Toilets that use pressure for flushing are traditional, and they still account for about half of all toilet purchases. A push toward minimizing the use of water when flushing led to the development of toilets that rely on gravity, which tend to have a quieter flush. In addition to these more common toilet types, there are also vacuum-assisted and power-assisted versions. These types of toilets also have fairly quiet flushes, but the vacuum types are generally less powerful than the other types.