The following is an overview of the spirits common to Seidr beliefs. Spirits appear similar to people, animals, or perhaps plants or terrain. However, they often have unusual features and/or strength and toughness which belie their size. They can form a psychic bond with a person, which grants each certain powers of the other and allows empathic contact even between the material and spirit planes. The benefits of the bond are different for each spirit, and are detailed in the description below.
A few very powerful spirits can cross directly over into the material world -- much as shamans cross over into the spirit world. Spirits who cross over into the material world usually appear as animals or people. However, sharp-eyed observers will notice odd features which set such spirit apart from other creatures, and anyone with second sight (i.e. who have travelled to the spirit world) can instantly recognize such spirits immediately for what they are. They generally have exceptional strength and toughness compared to usual creatures as well as the benefits listed for their spirit type.
The alfar are feared more than favored. The winter sacrifice of Alfablot is intended to appease them. King Olaf Thickleg was buried in a great mound, and afterwards people of the area worshipped him as Olaf the Elf of Geirstadir, with sacrifices and prayers asking for good crops and plenty. In Kormak's Saga, Thordis, a wise woman, explained that Thorward must sacrifice an oxen to the alfar at a certain hillock to cure his wounds.
The Disirblot is celebrated on the Full Moon at the beginning of winter (in mid-October), with ale, pork, apples, and barley. The Alfablot is in mid-winter.
The landvaettir were spirits of a particular place. Old Icelandic law said that the dragon prow of any approaching ship had to be removed so as not to offend the landvaettir. Bjorn dreamt he made a partnership with a vaettir, and a strange buck joined his goatherd, which increased quickly afterwards. Viewers with second sight could always see many of the invisible spirits about Goat-Bjorn. Another time King Harald Gormsson sent a powerful Finnish shaman on a spirit journey to spy on Iceland, but the vaettir of the land appeared in beast form and drove him off.
The disir were connected to families rather than to places. The epic heroes Handir and Sorli slew their brother and cried out, "the disir incited us to do this." Thidrandi, son of Hall of Sida in Iceland, was killed by nine women with swords dressed in black after converting to Christianity -- said to be disir angered by his ceasing the usual sacrifices and honors for them. They can withdraw their protection when angered, as they did to King Geirrod, to whom Odin spoke as he died and said, "The disir are angry." Thorgerd and Irpa, clan-goddesses of the Hladhir in Norway (from Njal's Saga) are powerful disir, the only ones referred to by name.