Rashi: A Ba'al Ov is a necromancer, who causes the dead to speak from his armpit (or from between the joints of his body or of his hands - Sanhedrin, Ibid.); ¹ whereas a Yid'oni is a sorcerer who places the bone of a wild animal (or a bird - Rambam) called 'Yado'a' ² in his mouth, ³ which then proceeds to speak.

Rashi: It pertains to the Ba'al Ov and the Yid'oni themselves. ¹

Seforno: It is a La'av against turning to Ovos (one should turn one's back to them) ? how much more so against honoring them.

Targum Yonasan: It pertains to those who use their services. ²

Sanhedrin, 65a and Yerushalmi Sanhedrin, 7:10: The Ba'al Ov and the Yid'oni themselves are Chayav Sekilah, whereas the person who uses their services is only subject to a La'av. ³

Rashi: It implies that it is not merely seeking them, but actually dealing with them, that contaminates a person and renders him contemptible in the Eyes of Hashem.

Seforno: It impliess that seeking them to contaminate oneself via their services is forbidden, but that Beis-Din are permitted to learn abour them in order to be able to judge sorcerers. ¹

Moshav Zekenim: It implies that someone who seeks them is Tamei and does not cleave to Hashem.

Oznayim la'Torah: It implie that, as opposed to the Tum'ah that comes from eating Tamei food, where one eats for the pleasure and the Tum'ah comes automatically, the Tum'ah of Ov and Yid'oni come when one deliberately seeks it. ²

Sifra: It implies that the spirits do not come upon a person unless he seeks them. ³

Oznayim la'Torah: Because the spirit Ov, which does not operate on Shabbos, is a testimony to the Shabbos. ¹

Refer to 19:30:1:5.

Rashi: It is as if to say 'Know, Who you are switching for whom!'

Moshav Zekenim: To teach us that one should seek [to know the future] only from Hashem. ¹