In the Bloodmoon main quest, the player starts by doing odd jobs for the Imperials' Fort Frostmoth on Solstheim. When the fort is attacked by werewolves, the player must travel to the Nord village of the Skaal at the north of the island. The player must then perform several rituals to be accepted into the village. The player is informed of the Bloodmoon Prophecy, a ritualistic hunt led by the Daedra Lord Hircine.
Unlike the first expansion Tribunal, which added a city separate from the world map consisting of interior cells, Bloodmoon adds a large new island to the original world map, a cold northern territory named Solstheim. Rather than the Dunmer (dark elves) that are the indigenous race in the nearby Vvardenfell, Solstheim is populated largely by the Nord race. This is largely because the island sits more or less astride the border between Morrowind and the Nord homeland of Skyrim. This expansion adds new enemies, the East Empire Company as a joinable guild, and the possibility to become a werewolf, akin to the inclusion of vampires seen in Morrowind. Bloodmoon also uses larger and more detailed environments, including snowfall, thus raising the computer hardware requirements.
Source: Wikipedia, "The_Elder_Scrolls_III:_Bloodmoon," available under the CC-BY-SA License.