What is the difference between an electronic signature and a 'wet ink' signature?

Electronic signatures can take many forms (see What is an electronic signature?).  

A ‘wet ink’ signature is where the parties to the document write (sign) their names with their own hands upon a paper document by ink pen.  Although some specific types of legal documents do still have to be signed by the traditional ‘wet ink’ method, most documents including commercial contracts can be signed by electronic signature.  If a traditional wet ink signature on a piece of paper is scanned in to an electronic device, the scanned version is an electronic signature.