The two words are very similar, but they have a different meanings and cannot be used interchangeably.
"Confirm" has two meanings: by confirming something, we can give our approval or our consent to an action taken by someone else. It can also be used to strengthen an opinion, or to assure an appointment or an order.
"Affirm" means to validate something or to state positively. In legal English, an appellate court can affirm, or, in other words, uphold a judgment, which means that the judgement of the lower court is kept in place. Another example in legal English is when a minor reaches the age of majority and affirms, or disaffirms, the contracts such minor had made with other parties.
In conclusion, confirm is an active assurance of something said or done before, while "confirm" is simply leaving it in place without interfering.
There is a grammatical difference between the two words as well: affirm is used in affirmative sentences (for positive statements), whereas confirm can be used in both negative and affirmative sentences.
