Qi gong refers to a multitude of physical and mental exercises for gathering Qi and learning to direct its flow. It enables us repair and maximize our health, learn about ourselves and about how our Qi is influenced by the Qi of the world at large.
Qi gong exercise involves breathing, meditation, and movement of all kinds. It is done in standing, lying, and most other positions the body is capable of. There are literally thousands of Qi gong exercises. A google search will turn up hits on many different types of exercise sets with as many or more different names.
In my practice I sometimes teach a patient an individual set as therapy for a specific condition. The best way to study is to take up a complete system. If your first priorty is physical or mind health you may start with individual exercises from your chosen system to address those concerns. They are in fact, the first priority in the study of Qi. Achieving the ability to use Qi for medical, martial, or spiritual ends requires a fundamental level of physical and mental health.