I love doing yoga, it is so relaxing and calming!  I especially like Hatha yoga and Rāja yoga
That's great! What exact are Hatha and Raja yoga? What are the differences?
Rāja yoga is sometimes called "royal yoga" or "classical yoga" while Hatha yoga is mainly supplementary physical techniques within the broader concept of Yoga.
I'm not very familiar with yoga. What is the basic concept or classical approach?
I'm not sure, but generally yoga is a group of physical, mental, and spiritual practices
When did it become so popular? I know so many people who enjoy it.
It became popular in the 1980s in the Western world.
Where did it originate?
to pre-Vedic Indian traditions; it is mentioned in the Rigveda, but most likely developed around the sixth and fifth centuries BCE, in ancient India's ascetic and śramaṇa movements. The chronology of earliest texts describing yoga-practices is unclear, varyingly credited to Upanishads. The "Yoga Sutras of Patanjali" date from the first half of the 1st millennium CE, but only gained prominence in the West in the 20th century. Hatha yoga texts emerged sometimes between the 9th and 11th century with origins in tantra.
Yoga
. In Hinduism, practices include Jnana Yoga, Bhakti Yoga, Karma Yoga, Laya Yoga and Hatha Yoga.
Traditions Classical yoga.
What is often referred to as Classical Yoga or Astanga Yoga (Yoga of eight limbs) is mainly the type of Yoga outlined in the highly influential "Yoga Sutras of Patanjali." The origins of the Classical Yoga tradition are unclear, though early discussions of the term appear in the Upanishads. The name "Rāja yoga" (yoga of kings) originally denoted the ultimate goal of yoga