Is Taiji, Qigong? What is the difference between Taiji and Qigong?

Taiji is the name of the symbol commonly called the Yin Yang:

Taijiquan is a martial art, with some of it's principles based on the Yin Yang theory. I does contain many movements that could be considered Qigong. However, the higher level practitioners I have met all practice Qigong separately. There are specific Qigong used by various Chinese martial arts, but Qigong is not a martial art.

Practicing Taijiquan for health improvement is the equivalent of practicing western boxing for health improvement. All exercise can help with your health, but Qigong is the more specific system.

There are a number of groups or people who teach Taiji for health. They leave out the martial components. Again, this is like being taught boxing, but moving slowly, without hitting anything, no gloves, no opponent. It won't make you a good boxer, and you don't get that much health improvement either.