You actually determine this, though generally 0-9 Yards is 'Short', 10-15 Yards is 'Medium' and 15+ is 'Long'...
however, this varies for everyone, even in the NFL. Some will say a Short Pass is 0-7, a Medium is 8-12 and a Long 12+.
Other will say a Short is 0-10, a Medium 11-20, and a Deep 20+.
The Offensive System will sometimes dictate this... what is Medium for a West Coast isn't always Medium for a Run and Shoot or a Spread Vertical.
Generally speaking though, look at Short as 'Under 10 Yards' when talking about Pass Distance, and remember that's from the Line of Scrimmage, not the QB. A Medium can be viewed generally as 10-15 or 10-20 then, and anything past 15 or 20 is Deep.