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They're a little bit like a notary public, in US Jurisprudence. Sort of. An oracle could even do the same job, if that's what the requirement is for the particular application. "This individual proved their identity and signed this document, stating the document was true."

a high tech ninja notary public. I like it!!!
That is how I will explain them from now on.
XD

Yeah, before I can address @ned's question I need to first know what an Oracle is.