You probably don't realise it but when you are playing you hold your pick (or plectrum) in a million different ways. The angle between the pick and the strings, the length of exposed pick and how hard you squeeze the pick all affect the sound you will get. How you hold your pick eventually becomes a sub-conscious thing: difficult to describe but crucial to the sound you get.
A good description of this comes from B.B. King , the master of expressive blues playing, who described his pick technique like this: