Odin was chief of the Aesir godswho were enemies with the jotunn (giants). Loki was son of the giant Farbauti and became an "ally" of Odin, often tricking and working against the Aesir, sometimes to save his own skin.
Loki's children with Angurboda, a giantess, were Fenrir, a giant wolf, destined to kill Odin, Jormungandr (the Midgard Serpent), destined to kill and be killed by Thor (Odin's chief son) and Hel, a goddess born with half her skin decayed.
Odin and the Aeisr banished Hel to Niffleheim, threw Jormungandr into the ocean and let it encircle the human world (Midgard), tied up Fenrir and bound Loki in a cave.
The Aesir and Vanir gods then waited for Ragnarok, when Loki, his children and their giant kin would do battle with the gods and destory the Earth.