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.
In most jurisdictions, the crime of forgery is not charged unless the forgery is done with intent to deceive or the attempt to commit fraud or larceny.
For example, works of arts can be copied or replicated and no crime be committed unless someone attempted to sell or represent the copies as originals. Then the copies would become illegal forgeries.