Shields block all damage from an incoming attack. All physical damage, all piercing damage, all elemental damage, all vitality damage, any health reduction attack, quite literally EVERYTHING. If you have 100% block chance, and 100% reduce shield cooldown time, you can be nearly invincible.
Believe me you, running away is manditory in this game. I also fell into the idea that because I build a "tank" character, that I should be able to charge in and pwn everything.... but it is simply not the case. Feel free to learn this lesson for yourself, though. <shrug>
And, petmasters can manage thru latter Act 3 and 4 with a full pet setup, it takes a LOT of patience, careful control of your minions, and "pussyfoot" tactics. Oh, and re-summoning wolves a lot Not impossible by any means of the imagination.