The difference is the same of the Chinese pronouns NI (familiar form for the pronoun you) and NIN (respectful or deferential form for the pronoun you). "Mi scusi" is used when you want to show respect or when you don't know well someone (especially when the person is older than you but not only). "Scusa" or "scusami" when you already know well someone or if you are close friend with the person you are talking to. Both ,"scusi" and "scusa" are imperative verbal form. They mean "may you excuse me" and "can be you excusing me". In Italian we use the third personal feminine pronoun "lei" to show respect, both with men and women. This is because in Italian the term "person" is feminine as grammatical gender. For exemple: as respectful form "lei" means that I don't speak directly with you but with your person, and person as noun is feminine in Italian, so I would say " Mister Smith LEI (She) is right" etc etc. or "posso farLE una domanda?" (May I ask "her" [to your person] a question?" "-le" after the verbal stem means "to her". So "mi scusi" actually means "may your person (she/her) excuses/pardons me". "MI dispiace" hasn't got respectful or familiar meaning and it can be used in both the cases: it means "I am so sorry". If you ask some information you should use the form "mi scusi". It is more formal and polite.