Yoga is generally a system. However, it’s not one of beliefs. The discipline is a group of techniques that provide guidance for an enriched standard of living. Quite a few source texts are behind all of this, but there are two that are better known than all others. The first are the Sutras, and the second is the Bhagavad Gita. Both of these explain fulfillment and higher awareness, not just in terms of the nature of them, but also the obstacles preventing their attainment. Most importantly, they cover a number of methodologies for the attainment of said goals.
As with any large and particularly ancient body of knowledge, there are certain aspects of all this that are just far too subtle to get picked up from lectures, books, or videos. In fact, the only way to learn them is through experience. This discipline has a long-standing emphasis and tradition of a student-teacher relationship, where a teacher guides a student through their own practice as they find a deeper understanding.