Hand of Protection is a core paladin ability learned at level 48. As of Patch 3.0.2 this ability replaces the former [Blessing of Protection] and works in the same way, but uses the Hand system to prevent conflicts with Blessings. Players often use the old acronym "BoP" for this spell, even though it should be "HoP". In Legion name is reverted back to "Blessing" instead of "Hand" probably due to popular use of "BoP".
This does NOT protect the target from spell damage. This should not be used as a last minute protection if the target is being hit by spells (such as anyone running from a Mage in PvP).
Unlike Divine Shield, this does not remove all preexisting debuffs (only physical like Hamstring), so it will NOT entirely double as a Hand of Freedom.
Against rogues, if they have any stacks of deadly poison on the target you protect, a rogue can still use Envenom to damage the target because it is a Nature damage attack, not physical (its a rare thing but it can interrupt something like a bandage or be potentially lethal if you aren't careful.)
Players protected lose aggro while under its duration. Tanking paladins can use this as a last resort method of aggro management, or because of a player's inability to perform melee actions through the shield, an effective way to reign in a melee dps class who is unable to control their own aggro. The threat reduction only lasts for the duration of the hand however, so a tanking paladin needs to make sure they are able to generate enough threat to keep aggro when the hand expires.
The paladin Judgements are considered to be physical abilities and cannot be used while protection is active.
This does protect you from fall and fatigue damage.