The difference between being nostalgic and wistful

If you are nostalgic, you look fondly back to an earlier point in time. Nostalgia is wishing for a return to the way life was in the "good old days." Nostalgic people tend to remember the past as better than it actually was, regretting the good aspects of old times that are lost and forgetting about the bad aspects that perhaps we are glad to have lost.
To be wistful, on the other hand, is to wish or yearn for things to be different. It could include a longing for the past, but it could also include a desire for a better future. It's a more general longing than nostalgia. It could simply be longing—being wistful for—finding that perfect mate when you see a happy couple get married.
Both terms carry with them a tinge of melancholy or sweet sadness.