While Wu Shu and Kung-Fu are terms that describe all of the Chinese martial arts, they are two different things. While Kung-Fu is the term of all Chinese martial arts, WuShu can basically mean any martial arts style of the world. As an alternative, I could say my "WuShu" is Karate. Or my WuShu is Muay Thai, Chakibo, Brazilian Jiu-Jitsu, Capieora, Sambo, etc. That's Wu Shu in the broadest sense of the term.
The big difference between the two is that Wu Shu is also a sport. You have your Comtemporary Wu Shu which is the sport and Traditional Wu Shu. Traditional refers to the multitude of arts before the establishment of the Communist regime that became the People's Republic of China. But in the most notable sense is that Wu Shu is a sport while Kung-Fu isn't meant for sport competitions.
Sport Wu Shu is more of series of events. Such as one one one matches, katas, and gymnastics competitions. They're very popular throughout Asia, North America, and Europe. They have events for bare-handed styles such as the styles of Long Fist, Shaolin Fist, and Tai Chi Chuan. Then they have sparring matches and demonstrations with daos (single-edged swords), jians (double-edged swords), nandaos (southern single-edged swords), and so forth.
Other routines present in events are demostrations of Baguazhang (Eight-Trigrams Palm), Baji Quan (Eight-Extremes Boxing), Cha Chuan (Cha Fist), Chuo Jiao Quan (Poking Feet), Hou Quan (Monkey Fist), She Quan (Snake Fist), Piqua Chuan (Chop-Hitch Fist Boxing), Tan Tui (Spring Leg), Tong Bei Quan (Through-the-Back Fist), Xing Yi Quan(Heart and Mind Boxing), Wing Chun, Zui Quan (Drunken Fist), Sanhuang Paochui (Cannon Boxing), and so forth.
Wu Shu mainly sport Wu Shu is merely show for the grace and beauty. Traditional Wu Shu is basically all the styles not for show before the rise of Communism. In a sense, Wu Shu is the modern form of Chinese martial arts.
Whereas Kung Fu is referred to all of the Chinese martial arts. Original meaning is directed towards your expertise in whatever skill instead of the style. For example if you're a good fighter, it means your Kung-Fu is great. If you need improvement, your Kung-Fu needs improvement. In the past, all the styles were referred as "Chinese Boxing." Meaning each style of Kung-Fu was a "fist" or Chinese Boxing style.
Zui Quan can either mean Drunken Fist or Drunken Boxing. Today while Kung-Fu could mean a number of various Chinese styles, it was the term of a person's training such as strengthening the body and mind. It was about learning and perfection of skills.
In short, your Kung-Fu was your attack power, speed, fortitude, adaptability, reaction, and so forth. Kung-Fu meant those attributes than the style. Kung-Fu can be implemented outside of martial arts such as driving. If you're a good driver, your Kung-Fu is good in driving. If you're bad in bed, you could say your Kung-Fu needs improvement in bed. Your Kung-Fu was your skill in an area.
I happen to be a martial arts expert so I can say my Kung-Fu is good in martial arts. I'm also a marksman with an assault rifle back in Army Basic Training, so I can say that my Kung-Fu is good in firearms. I'm also an actor, so I can say that my Kung-Fu is good in acting.
That's the main differences. While Wu Shu means the styles, Kung-Fu can be referred to the various Chinese styles but mainly your skills in respective disciplines.