Hash is THC crystals (trichomes) from a marijuana plant that is compressed with some sort of heat. You can even get simple hash by handling freshly picked sticky buds and rubbing your hands together. The friction creates heat, and that makes the hash form in your hands. There is no plant material in hash, even though it's dark--some types of hash are light too.

Hash has a higher THC content than marijuana, but I can't comment on the kind of high it gives. Maybe someone who's had good hash can.