Tomb Kings do not have to be a horde. They have one of the best "Monster" lists in the current line of books. In fact, most of the Tomb Kings lists that perform really well have shockingly few models on the board.Tomb Kings
Overall: A horde army relying on numbers and magic, I think. Sadly with the new book coming out and having only played a few games against it I can not make a good judgement here.
Offensive/Defensive: Probably more defensive, from what I've seen they seem to lack a real punch unit. I must admit confusion there.
As for lacking "real punch" units... they have Tomb Guard, War Sphinx, Chariots, and Necropolis Knights. Tom Guard are fantastic, relatively cheap, good stats (for undead), and killing blow. War Sphinx with breath weapon can kill an absurd amount of models in a single phase (I think it's somewhere around 32-36 models in the phase it breaths if everything hits and wounds). Necropolis Knights offer up a load of attacks as well.
Tomb Kings are either an agility force or a horde force. Horde force plays more traditionally but can be a bear to win with without the right build. Agility Force revolves around putting your power blocks where they need to be when you need them. They DO rely on magic more heavily than most other armies.