Ward is a priest-specific mechanic identical to that of Magic: the Gathering's protection.
The basic concept: Here's a new Priest card called White Knight.
This minion cannot be selected by Warlock cards (e.g. it can't be attacked by Warlock minions, it can't be a target of Bane of Doom, etc.) The mechanic seems fun and powerful, and I think it would make an impact on the game. It would also be interesting if a 2 mana 3/3 was designed with Ward: Priest, so that way it couldn't be healed or applied buffs. It just seems like a fun mechanic and it would impact the game without being OP. There could also be Wards for specific tribes or even an all-encompassing ward, which would be identical to the protection from everything mechanic from MTG.
I'm not sure. I never played Magic, but honestly, were the protection cards really ever used? And even then, Hearthstone has 9 classes compared to Magic's 5 colors (not to mention at least Magic decks might run multiple colors).
Protection from your own class might be kind of interesting, though I could see it easily being under or overpowered (and already that's fairly similar to generic Spell Immune cards). Ward tribes makes more sense even if it might not warrant a keyword.
To quote me from the past, "It doesn't have to be good, it just has to be sweet."
It's just a cool concept, and to answer your question, protection cards were used when the game was budding. It's just another MTG mechanic to bring over and it could see play (e.g. a 3 mana 3/4 with Ward: [insert class here] for the Paladin).
I'd like the concept due to its meta stopping abilities. But I don't think it wold realistically see much play. It is one of those cards however that doesn't need to be played for it to be a threat so the meta is always a mix up and is never one sided to a single class.