What is the difference between tai chi and chi gong?

According to the Secrets of the Yellow Empereror, Tai Chi is one of the hard forms of Chi Gong (Qi Gong). Kung Fu is the other.

Traditional Qi Gong is a medicinal style with absolutely no fighting applications. While Tai Chi does have martial applications.

tai chi has some fighting applications sense its a martial art, qi gong doesnt

Chi Gong means "Chi work/practice" in Mandarin. Tai Chi is probably the most widely known type of Chi Gong and can be applied to use as a self defense.

Chi Gong can be Tai Chi, Standing meditation, etc.