The Æsir are one of the two pantheons in Norse mythology, Vanir being the second of the two. Technically, Thor is a member of the Æsir, while the real Freyja is actually one of the Vanir. Though, after the Æsir-Vanir war, the Vanir became a sub-group of the Æsir. And technically, áss (singular of æsir) means "god".
Well originally in simple terms the Pantheon of Norse is divided into Gods (Aesir, Vanir) and Giants (Frost, and Flame), Aesir tending to reside in the "heaven" Asgard where Valhalla is said to be and heroes are greeted, Vanir reside in Vanaheim, a nature based realm, Freyja falls under the goddess of beauty and fertility so she along with her brother Frey are nature based in their respective domains. Well she is technically Aesir and in this instance of SAO she was Thor is disguise who is without a doubt Aesir.
Furthermore Giants exist in Jotunheim and Muspelheim which represent the frost and flame giants respectively, and in numerous myths they come into conflict with the Gods. Thus in this series Jotunheim has evil gods in ALO, and we have the Caliber with evil giants.