What is the difference between turkey neck and a double chin?

While not true scientific terms in general I think of double chin as mostly excess fat in a younger person and a turkey neck as lose skin and lose platysma muscles in an older person.The operation for each is completely different and a consultation with a board certiifed plastic surgeon is a good place to start the conversation on what you may like to improve. 

 In general a turkey neck is related to loose skin and laxity of the platysma muscle. This gives a loose hanging appearance to the skin under the chin that looks like a turkey's. Double chin is related to excess fat underneath the chin causing a roll when the neck is flexed. Hope that helps!