Answer....when its an autograph. Very simply a signature is when a person signs a cheque, document or similar with what would be their signature as they have signed all through their life. 

An autograph could be almost anything put onto paper or a photo by the same person but not their signature. It might be their initials, it might be the scribble that most football players put onto photos when signing, or it may be one of those little messages that some difficult celebrity adds to your nice photo when you got them at a stage door "not for eBay!" 

So very simply a signature is a signature but could be called an autograph, but it should not be assumed that an autograph is a signature! 

Garry King