What is Kriya?
The word "kriya" is from the Sanskrit root "kri" which means "to do". It implies an action, rite or ritual, hopefully one that is performed consciously. So sometimes it refers to a technique like your pranayama or the shat kriyas, which are purification techniques. It is also used to describe and discuss the currents within the subtle body, sometimes called kriya currents.
Kriya Yoga is the only yoga discussed by Patanjali in his Yoga Sutras. In book II, sutra I, he states "Tapaha Svadhyaya Ishvara-Pranidhana Kriya Yogaha." This translates "Kriya Yoga is self-discipline (tapas), self-study (svadhyaya), and attunement to Life."
Kriya is a system which deals directly with the mind, its structure and dynamics. Embodied within it is a mystical psychology which offers a profound recognition of how we create, sustain and dissolve the circumstances of our life. Its unique approach is rooted in its focus on karma, the laws of causation, and how karma is generated, activated and can be softened and modified. Kriya is a way of self-study and meditative attunement which induces a direct perception or insight into the nature of consciousness, the pattern of your mind and of life itself.
Kriya encompasses all eight limbs of the classical yoga system as presented by Patanjali in his work The Yoga Sutras. Patanjali's Yoga Sutras, as you may know, is a text that codified what was already an oral tradition for thousands of years before it was committed to writing over 2500 years ago. One of the primary and fundamental limbs of the Patanjali system is Hatha Yoga, the ability to form and hold a still posture while regulating the breath. The implication is that to sustain physical and mental health, a stillness of the body and mind are essential. Without this fundamental skill and the benefits of its practice, deeper and advanced yoga techniques are difficult to master.