As defined by OregonLawHelp.org.pdf), an affidavit is a written statement that is notarized. If you need to provide a written statement from someone with personal knowledge about what you are trying to prove to DHS, and this person cannot testify in person, you can use an affidavit.
Affidavits are not hard to prepare, but they do require certain pieces of information. The information in an affidavit mustbe first-hand information. It cannot be second-hand informationor speculation. Affidavits can be written in your own handwriting or typed.
Here's how to write an affidavit.pdf) from OregonLawHelp.org.
Find some of the more common types of affidavit and learn what they're used for at Rocket Lawyer.