Decrement or a variable.
x--;  // decrement x by one and returns the old value of x
--x;  // decrement x by one and returns the new value of x

x: an integer or long (possibly unsigned) The original or newly decremented value of the variable.
x = 3;
y = --x;     // x and y both contain 2

x = 3;
y = x--;     // y still contains 3, while x contains 2