Direct object pronouns are pronouns that represent the nouns that are directly acted upon by the verb. Indirect object pronouns stand for the noun that is the recipient of the verb's action.
As for formal VS informal, usted is the keyword. Whereas in english language being formal/polite has to do with the use of certain words, using usted (you) in spanish changes verbs in the sentence and makes the communication formal. Tú (you) is used informally. Usted is usually used for elders or those in higher up positions, like a teacher. Tú is usually used for friends.
