Piston valves are the most common type of valve on brass instruments. They consist of a cylindrical piston that moves up and down inside a cylindrical casing. When the piston is moved down, it opens a port that allows air to flow from the mouthpiece to the instrument. When the piston is moved up, it closes the port and stops the flow of air.
Rotary valves are less common than piston valves, but they are found on some brass instruments, such as the tuba and the euphonium. Rotary valves consist of a cylindrical rotor that rotates inside a cylindrical casing. When the rotor is turned, it opens a port that allows air to flow from the mouthpiece to the instrument. When the rotor is turned back, it closes the port and stops the flow of air.
Both piston valves and rotary valves have their own advantages and disadvantages. Piston valves are generally easier to play, especially for beginners, but they can be more difficult to maintain. Rotary valves are more difficult to play, but they are more reliable and require less maintenance.