Various terms are used to describe unofficial coinage, but I believe that there are distinctions. "Unofficial" is a good blanket term for any coin that was not struck officially-- but there are different categories-- counterfeits and imitations. Counterfeits, which closely mirror official coinage, were made to fool people and circulate as official, while imitations (commonly called "barbs") vary in style and range from fairly close to official, to coins that one might have difficulty even figuring out the prototype.
I have a page on this topic, with an emphasis on VLPP imitatives.
http://www.constantinethegreatcoins.com/barb2/
