What is the exact difference between these two stylistic devices: (1) Synecdoche (2) Pars-pro-toto ?
You're correct in the deduction you made in the penultimate paragraph of your reply to the first answer: pars-pro-toto defines a part used to describe the whole, and synecdoche "can refer both to this and its inverse of the whole representing a part."
So, an example of pars-pro-toto would be someone saying "Nice wheels" to refer to another person's car. The car is made up of more than its wheels, but this part of the car can be taken to signify the whole thing.
The above would also be an example of synecdoche, however, synecdoche can also consist of the opposite effect, that of the whole being used to describe a part. An example of this could be referring to members of the police force as "the law," as police officers are only one part of the entire system of law enforcement.
Synecdoche is also sometimes confused with metonymy, which is when the concept of an object is used to describe it. For instance, "a pack of smokes" equates to a pack of cigarettes; cigarettes create smoke, therefore they have come to be described by that correlating idea.
These tropes (pars-pro-toto, synecdoche, metonymy, and merism) are easily confused as all refer to an object being described by something other than its proper name. Just remember, pars-pro-toto means "part taken for the whole" and synecdoche can refer to either a part taken for the whole or a whole used to describe one of its parts.
Hope that helps!
Latin for "a part (taken) for the whole," pars pro toto is a literary term which a portion of a place, object,or concept represents the entire place, object, or concept. It differs from a merism, which refers to a whole by means of an enumeration of parts; for example the expression "lock,stock,and barrel, which are all the parts of a rifle represent the complete whole of something; metonymy, in which a place, object, or concept is called by something or some place associated with that place, object, or concept. For example, in his poem "Spring," Richard Hovey writes,
I said in my heart, "I am sick of four
wall and a ceiling.
I have need of the sky...."
and uses "sky" for open air and space. Another example of metonymy occurs when people refer to the executive branch of the U.S. government as "The White House."
An example of pars pro toto is the use of "Great Britain" for the entire United Kingdom of England, Ireland, and Scotland. This usage and also the use of the UK for England alone is imprecise and can be offensive to some, as well. For, the United Kingdom [UK] includes not just England, but also Ireland, and Scotland, as well.