An Affidavit is a legal document that basically swears that a certain statement is true. A person can write it when s/he needs it to testify for him or her as evidence, or when s/he needs to legally declare something, such as a lost license or passport. The author signs it in front of a lawyer or any other taker of oaths and the said document can be used for any legal purposes that it was written to serve. Different kinds of affidavits are used all over the world for all sorts of purposes. There are many general templates found on the internet for the more general ones and you can always ask your lawyer or nearest notary public for assistance too.