I'll tell you streight up whats what!
Stats (except for attack and any extra stats aside from the basics) only work while the shield is up. You can block while casting. You can block with your back turned.
Def = Def added while shileds up vs NORMAL MELEE ATTACKS
Attack = Damage done with shield bash/II
Blocking = Blocking rate (in %)
Weild rate = Time you will keep your shield up (in seconds)