An affidavit is a document that certifies that the information you provide is true. It's similar to making a sworn statement in court, but in written form. Affidavits require a signature to be legally binding.
There are several different categories of affidavits, including forms for financial applications, business lawsuits, estate planning, divorce proceedings, child support matters and immigration applications. The details differ, but in every case, you swear that the information you provide is correct.