Short answers, pulled from the story.
Old Norse religion, also known as Norse paganism, is the branch of Germanic religion that developed among the North Germanic peoples and is the best-documented of all the forms of Germanic religion. Unlike Christianity, it had no codified doctrine or professional priesthood; its practitioners called it forn sið (old custom) or heiðinn sið (heathen custom), emphasising ritual behaviour over fixed belief.
The most widespread deities were Odin and Thor. The pantheon divided into two groups: the Æsir, whose major figures included Thor, Odin, and Týr, and the Vanir, which included Njörðr, Freyr, and Freyja. Loki's status was problematic; sources including Vǫluspá and Snorri place him imprisoned beneath the earth until Ragnarok.
Blót was the primary religious ritual in Old Norse religion, consisting of animal sacrifice. According to the Saga of Hákon the Good in Heimskringla, animals were slaughtered and their blood, called hlaut, was sprinkled on altars and on the inside and outside walls of the temple; ritual toasts were then drunk at a sacrificial feast. Blót could also take place before duels, before sailing, after trade concluded, and at funerals.
Seiðr was a form of magic and divination practised mainly by women called vǫlur and spæ-wives, often in a communal gathering at a client's request. It was associated with the Vanic goddess Freyja, who according to Ynglinga saga taught it to the Æsir. Ninth- and tenth-century female graves containing iron staffs have been identified as those of seiðr practitioners, and scholars have debated whether the practice derived from Sami or Balto-Finnic shamanic traditions.
Christianity spread gradually through Scandinavia via political conversion; the Danish king Harald Klak converted in 826 to secure alliance with Louis the Pious, and the Swedish kings had converted by the early 11th century, with Sweden fully Christian by the early 12th. Iceland reached a legal compromise at the Althing in 999, adopting Christian law while permitting private pagan sacrifices. Mass conversion driven by social ties, rather than individual conscience, was the norm across the region.
Yggdrasil is a giant ash tree at the center of Norse cosmology; the name means "Oðinn's Steed", combining Yggr (a synonym for Odin meaning "the terrifier") and drasill (a poetic word for horse). According to Grímnismál, the gods meet beneath it daily to pass judgement, a serpent gnaws at its roots, a deer grazes from its higher branches, and a squirrel runs between the two exchanging messages.