/shield is roughly 30 second duration, with 3 minute reuse.
It will reduce the damage taken by the /shield target by 50% and will give 75% to you. The amount of damage taken to you can be mitigated further using a shield in the offhand. No numbers on this however to really see the effect. (Just going by what it tells you).
Warriors get this ability around level 30ish, can't remember exactly as it's been awhile.
Rooting mobs in melee *can be bad (from a tank's perspective). Rooting makes the mob ignore all forms of aggro generation (while rooted) and attack whoever is closest. Of course, this can be compensated for by everyone attacking from further away from the tank, but it gets tricky. Luckily, cleric root is notoriously short lived ;). (Root also messes up any movement/positioning that the tank has to do during the fight). Of course though, if it's life or death, by all means root :).
*the use of the word "can" means that (as sense would say) it's not always true...just can be ;)