In Patanjali’s Yoga Sutras, asana is defined as a "steady, comfortable posture." The sutras do not specify any particular poses, but require simply that in practicing asanas, a position which is both steady and comfortable should always be sought.
Historically, texts and teachers have described different numbers of asanas. The classic texts of Hatha yoga refer to 84 asanas as taught by Lord Shiva, often with the first four of these being considered necessary to achieve spiritual perfection. These are siddhasana, padmasana, bhadrasana and simhasana.
Other teachers and texts have suggested that there are an infinite number of possible asanas, or as many asanas as there are beings. Sri Dharma Mittra, a yoga teacher well-respected by the contemporary schools of Iyengar, Ashtanga Vinyasa and Sivananda yoga, catalogued a list of 1,300 yoga asanas.
It is recommended that asanas are practiced with an empty stomach and without using excessive force or pressure. Asanas can be combined with pranayama practice to enhance the benefits of the poses. They should always be practiced with mindful awareness, uniting the body, mind and breath. Specific asanas can be practiced to help alleviate specific health problems or physical issues.
There are many benefits of asana practice in general. Asanas help increase flexibility and strength. They stimulate all of the physiological systems of the body, including the circulatory, immune and digestive systems. They help develop the mindfulness and focus needed for meditation. On a more subtle level, they are said to stimulate the energetic body, opening the chakras and the nadis.