Hoid is a recurring character—he's been in all of Brandon's Cosmere books (Elantris, Mistborn, Warbreaker, Way of Kings). Usually, his appearances amount to cameos, and we saw a lot more of him in this book, but confusion and mystery surrounding him is the norm.
We know that he is quite old, that he was present at the Shattering, has the ability to move between worlds, and likely possesses a large number of unknown powers. We know he's one of the good guys, opposing Rayse (Odium), who he knew from before the Shattering. He seems pretty untouchable by anyone who's not a god, which is probably how he gets away with acting like a prick.
A while back, there was a "Svudu cage match", which is where a bunch of nerds get together and argue about whose character could beat up the most other characters. When it came time for the finals: Rand al'Thor vs. Jaime wossname (from those books by a guy named George or something), George saw the writing one the wall. There was no way Jaime would win in a straight-up fight. So he made a proposal: All of mine vs. all of yours. Bring your top five from not only the WoT, but any of Brandon's books as well, and I'll bring my top five.
Brandon's comment? "If I can bring Hoid, the J man's toast." As if Rand Al'Frickin'Thor weren't enough. The moral of the story? Don't mess with Hoid. Dude's got all kinds of crazy crap up his sleeve.