Asgard is the most powerful world is the Nine Worlds, having the strongest army, greatest access to magic and the greatest influence on diplomacy.
Asgardian legend and Earth mythology would state that at the beginning of the universe, the "cosmic cow" Auðumbla thawed the god Buri, grandfather of Odin and great-grandfather of Thor, free from an icy realm, about the same time the Frost Giant god Ymir was created. Buri created Asgard and proclaimed himself king of the inhabitants, the Æsir.
Buri's son Borr, father of Odin, discovered Yggdrasil, the "Tree of Life", a cosmos which binds together nine planets, and ordered that Bifröst be built to link the worlds to Asgard with a "rainbow bridge". Centuries later, Odin would fulfill the throne and to current times in Marvel Legends remain king.
The Asgardians would live on Earth for a while to teach the Germanic races, the Vikings, their religion and in result be worshipped as gods. After leaving the Earth be (after the first Asgard-Jotunn war), their names and tales would pass down into myths and legends.
The inhabitants of Asgard are the Æsir, or Asgardians, a race not unlike the humans of Earth. They are a brave race whose religion demands that death in the battlefield is the only way to access Valhalla, Asgard's heaven. Similar to Viking tradition, Asgardian children are tought to fight as warriors from a young age, and base their fighting on the belief that death is the greatest victory a warrior can achieve in battle.
The gods are the rulers of Asgard and the most respected. Direct descendants of Buri, they possess superhuman strength and, in some cases, power over elements, vision, mind and influence. When learning to fight, many of the younger gods such as Thor, Balder, Sif and Loki were sent to Earth for a year to learn among the Vikings.
Bifröst - A theoretical link between the worlds of the Norse cosmos. Heimdall is the keeper of the bridge.
Valhalla - The realm of the honoured dead, where angelic Valkyries bring the souls of fallen warriors. There, the warriors feast in the halls of Valhalla and train for the glorious prophecised battle of Ragnarok.
Ragnarök is the prophecised battle at the end of the Asgardian dimension which will allegedly destroy the Nine Realms. It is said that the beginning of Ragnarök will be made clear when Bifrost is destroyed by the Sons of Muspell.