24eps, one OVA.
Synopsis (w/ spoilers)
It's based upon time travel, and multiple dimensions. Plot is it's strong point, along with character development and execution.
Personally I'd say it's a psychological drama with some thriller elements.
I loved the anime, but thats just me, it may not be your cup of tea.
Also with the time aspect and all it can get complex at times and while I understood most of it I could understand if others didn't. I wouldn't be able to enjoy this anime as much is I watched it at the age of 15 or so. So if you're young my advice is wait until you can enjoy it to the fullest.
p.s. If you want full fleshed out details just wiki it.