This enethe called in Dark Souls? Looks like a huge dark hell spawn knight

According to the developers, Dark Souls is not a direct sequel of Demon's Souls but a spiritual successor. So the story are not related even though the mechanics are really close, and the game are made by the same director and producer.

From an interview of the game director Hidetaka:

"What we we want to clear up is this is actually not a sequel to Demon's Souls - we're treating this as a brand new game created by the same director and producer," Miyazaki-san told CVG in an interview.

"Again it is created by the same director and producer, so the base concept and themes are very similar. However, the story itself as well as a lot of the game features have been recreated so that we can present this as a brand new title."

It is the amount of damage reduced. For example, if a shield has a physical damage rating of 100, 100% of physical damage is reduced when blocking. This also applies to the other types of damage (fire, magic, lightning) that shields have ratings for as well.

This is not to be confused with the attack rating that all shields also have, which is the amount of damage the shield would do if you used them to attack instead of blocking (you can use shields as a weapon if you equip them in the right hand).

Note that if you ever feel like doing this, all shields deal strike damage.