An affidavit is statement of facts which is sworn to (or affirmed) before an officer who has authority to administer an oath (e.g. a notary public). The person making the signed statement (affiant) takes an oath that the contents are, to the best of their knowledge, true. It is also signed by a notary or some other judicial officer that can administer oaths, affirming that the person signing the affidavit was under oath when doing so. These documents are valuable to presenting evidence in court when a witness is unavailable to testify in person. Affidavits may preserve the testimony of persons who are unable to appear in court due to illness, incarceration, moving out-of-state, death, etc. Judges frequently accept an affidavit instead of the testimony of the witness and are used in place of live testimony in many circumstances (for example, when a motion is filed, a supporting affidavit may be filed with it).
An affidavit generally consists of statements of fact regarding the issue at hand, with a section at the bottom for the affiant to swear to the truth of the statements made and affix his/her signature, which is then notarized in a jurat. A jurat is the bottom part of an affidavit where the officer certifies that the document was "sworn" before him. Jurat notarizations are required for transactions where the signer must attest to the content of the document, such as all affidavits and pleadings in court. It is a certification on an affidavit declaring when, where and before whom it was sworn.
In executing a jurat, a notary guarantees that the signer personally appeared before the notary, was given an oath or affirmation by the notary attesting to the truthfulness of the document, and signed the document in the notary's presence. It is always important that the notary positively identify a signer for a jurat, as s/he is certifying that the signer attested to the truthfulness of the document contents under penalty of perjury. However, jurat notarizations do not prove a document is true, legal, valid or enforceable.