What is an affidavit used for?

According to the Legal Information Institute of the Cornell University Law School, one common use for affidavits is as a means of presenting evidence in court. An affidavit is a declaration of facts that is made willingly and given under oath.

The Free Dictionary explains that any affiant, by definition, makes an affidavit without having undergone cross-examination; this distinguishes affidavits from depositions, which are records of examinations of witnesses or parties. A deposition can either take place voluntarily or after the issuing of a subpoena. The basis of an affidavit lies either in the affiant's personal knowledge of the relevant issues or what the affiant believes she is able to state is true.