It didn't occur to me until the other day that I haven't exactly discussed what wushu is...
The terms Kung Fu (Gong Fu for you hard-core Chinese culture enthusiasts) and WuShu seem to be used loosely and the only clear difference in literal meaning.
Kung Fu - Achievement gained through work; Skill gained through training.
WuShu - martial art.
Kung Fu today will 98% of the time refer to traditional Chinese martial art styles (Hung Gar, Northern Shaolin Kung Fu, Taichi, traditional animal forms, etc...)
Wushu, today, will 95% of the time refer to the practice of contemporary(modern) Chinese martial arts in a performance sport (Long Fist, Southern Fist, weapon forms, etc...)
However, Kung Fu can still be referred to as being in part of WuShu because Wu Shu styles are heavily rooted in traditional Kung Fu forms, and visa versa because traditional Kung Fu forms can be competed in WuShu competition. In a way there is a very yinyang relationship between Kung Fu and WuShu, they're two parts of the whole that is Chinese martial arts and still have a little bit of each other within one another.
Most people however just see Kung Fu as what you learn to defend yourself and what Bruce Lee did, and WuShu as martial dancing and what Jet Li did.
There are different ways to divide the styles of WuShu and Kung Fu, all you Avatards will remember Sifu Kisu giving his little shorts on Nickelodeon on how he based firebending on Shaolin Kung Fu and waterbending on TaiChi, he gives a pretty decent basic run down.
Northern Chinese martial arts tend to emphasize long extended strikes, jumping techniques, speed and agility. I've heard different reasons why this is, one being that it gets so cold in northern China that they need to exercise their whole body to keep warm. Another reason I've heard was that because of the wide open geographical regions it was nesessary to learn how to cover great distances and how to fight long ranged combat with empty handed styles. Northern Shaolin kung fu and Long Fist(ChangQuan) are the dominant styles in Northern Chinese martial arts.
Southern Chinese martial arts tend to emphasize the upper body, strong stances, and compact strikes. This is because its warmer in southern China, warmer climate, more people. More people, more cities and towns require less space to fight. Styles like Hung Gar, Wing Chun, Southern Fist are some of the more popular styles of southern Chinese Martial arts.