The literal translation of Wushu is martial arts, while the literal translation of Kung fu is time spent with work. One of the biggest problems in understanding martial arts is the improper use of terms.Perhaps a clearer way to look at the term Kung fu is to express it as “squired skill level” So the term Kung Fu can be used to describe anyone who has a high level of skill in any endeavor, but it takes on a special meaning in the world of martial arts.As a result of many films and television series the continued use of the term Kung Fu has simply come mean Chinese martial arts for most people, even some Chinese use this term to refer to martial arts, but this is a result of popularity of the word, mostly from Hong Kong films.
The relationship between the two terms can be more specifically looked at as Wushu simply being the root, and kung fu is a proficiency of the skills therein. I.e. the ability to apply those techniques as a result of diligent study, practice, and applied experience. With the factors one can be said to have Kung Fu.