From what reports say, it appears that a man took people hostage, poured gasoline on them, as well as around the building, and set the place on fire.
As of writing this, 33 are dead with many injured.
If anybody is interested in helping the families of this, there is a GoFundMe link below.
Kyoto GoFundMe
Original Story:
Kyoto Animations Arson
More related to the story, as well as the suspect. (From Melphina. Thank you)
Kyoto Animations Arson #2
Edited 7/20 to correct a misuse of the word "Terrorist". Also added a link with some more details not in the first article.