Piston Valves vs. Rotary Valves- What’s the Difference?
A Little Bit of History: Before the invention of slides and valves to change pitch chromatically, early brass instruments were fairly limited. Trumpets and horns were only able to produce notes within the harmonic series, in other words those produced naturally by a single length of tubing. Players could swap the ‘crook’ of the instrument to change the length of tubing and play a different set of notes, but playing chromatically was not possible. Horn players could also change the pitch by placing a hand inside the bell, known as ‘hand-stopping’, though this also vastly changes the timbre of the instrument. When valves first began to be introduced in the 19th century, they changed the game for brass players by offering them a full scale of notes to play with, vastly increasing the expressive freedom of the instruments.
The Technical Bit: Piston Valves and Rotary Valves both do essentially the same thing, but achieve it in a different way. When the valve is pressed, it reroutes the air through an additional length of tubing, lowering the pitch by a set amount. A spring then returns the valve to its original position.
Piston Valves: Piston Valves achieve this effect by moving a cylindrical stock inside a cylindrical casing. There is plenty of ‘travel’ in these types of valves, making ‘half-valving’, a technique most often employed by jazz players, easier than with Rotaries. Piston valves also produce a cleaner and more definite movement between notes, making them ideal for quick passages where each note needs to be heard clearly. 
Rotary Valves: Instead of moving up and down, the stock inside a Rotary Valve rotates through 90 degrees to redirect the air. Rotary valves produce a smooth transition between notes making them ideal for playing lyrical passages like those found in classical music. Rotary Valve instruments are said to produce a broader, mellower sound than their piston counterparts. These factors make them ideal for the French Horn, which is often used in legato passages in Symphonies. Although the most common instrument to use Rotary Valves is the French Horn, Trumpets, Flugelhorns, Euphoniums, Tubas and other brass instruments are also made in rotary valve variations. These instruments are rarely seen in the UK but have gained more popularity in Austria, Germany and Eastern Europe. 
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