The world of Mundus and Oblivion works through consent, though the mechanics of it are quite vague.

Conjurers are highly advised not to engage in communication with any daedra because this is the primary medium of providing consent. Never acknowledge the opinions, advice or machinations of conjured daedra. When engaging in conversation, fail-safes are a must to protect the conjurer from harm. Never unbind or summon an unbound daedra. The most important of all is to never conjure them with borrowed power (magicka providing apparels) but rather, use only the inherent magicka reserve.

On the other hand, a conjurer can command a daedra by manipulating him through magicka. As to how, it's magic.