I don't really understand TES lore but there has been few books with demons on them, and there fire demon i belive in arena? i know that deadras are not demonic then what are demons? Can some one explain this to me? i know that they have something to do with dagon.

"Demon" isn't so much a proper noun in TES as it is adjectival; Daedra aren't strictly "demons" in the proper sense, but they are "demonic", fitting the definition of an adversarial, diabolical spirit or fiend, and as such men frequently call them demons.
The Fire Demon in Arena was pretty surely a daedra; perhaps a fire atronach.

More uncertain is the nature of the "Snow Demons of Kamal", in Akavir. But it's generally understood that when someone says "demon" in TES, they're referring to daedra.

Demon's are featured in Redguard lore as well I believe, but in that reference their more like just malevolent spirits like wraiths.

Demon would appear to be an Imperial or just otherwise Mannish concept.