A notary public is an officer who can administer oaths. The notary makes official documents out of normal paperwork. An affidavit is a piece of paper until the person who signs it takes an oath before a notary public, under pains and penalties of perjury, that it is true. A deed becomes an official conveyance of land when the seller signs in front of a notary.
Centuries ago there were no computers, photocopiers or typewriters. "Scribes" actually hand wrote new documents. They were appointed by the king, or a mid-level yes-man.
Fast forward to today. Each state has its own guidelines for becoming a notary. Usually one must be at least 18 years old. You need to have people sign your application, often a present notary or two, a registered voter, or even an elected official in some states. Most states require a squeaky clean criminal record. In New Hampshire, the Secretary of State reviews the application.
A Justice of the Peace can do everything the notary can do plus marry people. The J.P. can also "swear out" a criminal complaint. Affidavits are defined as written statements, under oath, sworn to or affirmed by the person making it before some person who has authority to administer an oath or affirmation. That's where the authority of the notary comes in.
Potential clients sometimes ask me about some paper they wrote up without an attorney, often in the heat of a fight, and then tell me they want to enforce it "because it is notarized." Notarization only indicates that the person who signed it is who they say they are and that they signed the document for the purposes stated. Notarization does not magically transform an invalid piece of paper into a valid enforceable legal document.
Those old metal stamps that crimp the paper are no longer required in most states. A rubber stamp with the name of the notary, and the words "notary public", the name of the state, and the expiration date of the notary public's commission are included in the stamp. The stamp on the document is prima-facie evidence that the signature is genuine and that the person holds the designated title. In fact, the information can be typed, printed, or stamped. The same applies to a J.P. Rubber stamps replaced the old metal crimping devices, making it easier to transmit notarizations through copiers, scanners, e-mail and fax machines, which can not "read" the old fashioned raised imprint of an embosser.
Clerks at the registry of deeds used to trace over the old fashioned raised seals with a pencil so the imprint could be seen when copied. Most notaries who have the old metal stamps still use them because it gives an official look and feel to the document. That's fine. However, don't feel slighted if a notary skips the old embosser.
If a document is properly notarized, it's valid in all 50 states. This falls under the full faith and credit clause of the United States Constitution. Being a notary is relatively simple. You can learn it from a simple pamphlet they give you, or from an experienced notary. It used to be simple, anyhow. Back when Mitt Romney was Governor of Massachusetts, one of his official "acts" was implementation of a 15 page single spaced executive order specifying new notary guidelines. Why make things simple, when they can be complicated? As noted at the bottom of Mitt's order: "God save the Commonwealth of Massachusetts."