What's the difference between Nail Biting and Nail Picking...?

hi, i thought i was a nail biter, but my friend said no, he said i was a nail picker. He is a nail biter... what are the differences...? My finger nails dont look like http://hannahchan.com/nailbiting/wp-content/plugins/blonline/images/beforedamagestopbecomesirreversiblenailbiting.jpg but his do. im still confused on what a nail picker is, im assuming a nail biter is more extreme.