A form is a set pattern of blocks, kicks, punches, sweeps, and throws, which represent a fight against several opponents. Like sparring, forms are a different way of applying techniques you have learned in order to improve them, and to better understand your Kung Fu. To do forms well, the student must develop good stances, precision, power, speed, and intensity.
A form is not a dance, although they share similarities; it is a real fight against imaginary attackers, and a viewer with no Kung Fu experience should be able to tell the difference; s/he should almost be able to SEE the attackers.
Unlike martial arts styles like Karate or Tae Kwon Do, Kung Fu forms are not always obvious in the application of their techniques. In the old days, Kung Fu masters were greatly sought after by many people who wanted to be students, not all of them for good reasons. In order to protect their integrity and to stop their arts from being exploited, the masters would often hide moves within their forms. If a student eventually proved him/herself to the master, s/he would eventually be shown the true meaning of the move. This way, the masters could remain true to their art without running the risk of teaching deadly skills to unworthy students.