A poptart refers to a person whos musical, fashion and other tastes is dependant on what is popular at the time.
For example people whom only listen to Top40, throw out their CD's when they aren't in anymore and rate their clothes by which famous people wear similar styles not on what they like.
People who only like music made in the last 5 years are poptarts, also known as posers.
That guy dresses like a Goth but he listens to Simple Plan, what a poptart.
That girl only listens to INXS because they had a show on MTV, she is a poptart.