When you sign someone else's signature yourself, using that person's signature without authorization is called "forgery".
"Forgery" (also known as "uttering a false instrument") is a serious offense, punishable as a felony in all fifty states and by the federal government. It involves the making, altering, use, or possession of a false writing in order to commit a fraud.
The most common form of forgery is signing someone else's name to a check, but objects, data and documents can also be forged. Legal contracts, historical papers, art objects, diplomas, licenses, certificates and identification cards can be forged. Currency and consumer goods can also be forged, but that crime is usually referred to as counterfeiting.
The penalty for forgery, counterfeiting or, altering documents and instruments often varies according to the type of document altered, with important government documents at the top of the penalty list.
Learn more about Forgery Laws and Penalties at the following sites:
CriminalDefenseLawyer
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Millions of consumers are affected every year by identity theft. While many of these thefts occur mainly in the virtual world these days, there are still many criminals who try to pass themselves off as someone else by forging a signature.
You don't have to be a handwriting expert to be able to tell whether or not a signature is forged. The Law Dictionary shares a guide on How To Spot A Forged Signature.