What are Terafim?
Rashi: They are in the form of a person. Radak - some say that they are idolatry, like Lavan said "Lamah Ganavta Es Elohai" (Bereishis 31:30). Heaven forbid that idolatry would be in David's house! Some say that they are copper Kelim used to tell time, and help to tell the future via astrology. Michal would not put such a Kli in the bed in place of David! Ibn Ezra (31:10) says that they are human forms to receive higher powers. He supports this from here; Terafim were in the bed, and people thought that it is David.
Why did Michal put Terafim in his bed?
Rashi: It was lest they understand that he fled, and chase after him.
Malbim: It was so she could say that he is sick, and delay the matter, and David will have time to distance himself.
What is Kevir Izim, and why did Michal put it at the place of his head?
Rashi: It is skin of a goat flayed intact, like a pouch. It resembles human hair; Terafim do not have hair. Radak - Targum Yonasan and Shochar Tov 59 hold like this.
Radak: It is a pillow made from goats' hair.
