Different people answer this differently. Here is my take. Kung fu is the Romanized Chinese form of gong fu. They mean the same thing. Basically, in this context, any martial art originating in China. Technically, the Chinese apply the term gong fu to any skill you put a lot of effort and time into mastering, but using it to refer to Chinese martial arts in general is pretty well accepted. Wushu has two meanings, again depending on who you're asking. The more common use for it that I hear is in reference to the sport based on Chinese martial arts. However, it can be used as a synonym for gong fu as well.