The difference between the shield types is "Large/Tower" and "Small/Buckler."
The large/tower ones have superior block power, but no parry multipliers. You'll never parry with a tower shield.
The small/bucklers on the other hand have less block power, but they have a parry multiplier. If you time the block properly you parry, which can stagger the attacking enemy.

TLDR: Towers are for just holding block. Bucklers are for punishing combos.

The heart is actually not a heart, its a celtic trinity knot, but that's your stagger bar. If it maxes out you will be staggered.