Say Tai Chi and everyone has an idea of what it is, or at least has heard of it. Say Qigong and you are usually met with blank faces.
Actually, Qigong is a much broader category, and includes the modern version of Tai Chi under its umbrella. Qigong is usually thought of as a health practice involving work with the Qi, the Chinese concept of vital energy. Tai Chi, originally a martial art, is today practiced as a health exercise that focuses on the Qi. Very few people focus on the fighting aspects of Tai Chi any longer, so modern Tai Chi is actually practiced as a form of Qigong.