What is a notarized document and how do I get one?

...And also In order to get a document notarized do the people signing the document and the notary have to sign it at the same time or can I and someone else sign a document and I bring it to the notary and they notarize it?
Update: Instead of a notary could I just get a regular witness to sign it so there is the two signatures and the witness signing saying that they witnessed the signing of the document?
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