At one time, the use of the word asana was a reference to the original seated postures of yoga and mediation as well as the relation of one's spirit to a higher power. The basic definition of asana, derived from the Sanskrit word "to sit," is any one of the various positions of the body performed in yogic exercise. This definition is what most practitioners think of when they use the word today.
Western yoga exercisers are most familiar with the practice of asanas as a segment of one yoga path, Hatha yoga.
The History of Asana

The roots of yoga expand deep into the Hindu culture, going back thousands of years. However, Indian sage Pantajali is credited for establishing the foundation and inspiration for the classic belief and form of yoga as most of us know it today. Many cite his theorem within the Raja Yoga Sutras, based on the study of ancient yogis, as the modern yogi's guide to living.

Over time, people disregarded other parts of Pantajali's total philosophy, yet millions of yoga gurus and students worldwide still embrace the eight steps of the Yoga Sutras.

The total extent of yoga's philosophy of the asana is not explored when it's simply considered a posture. Combined with pranayama, asanas are stepping stones to the ultimate goal of enlightenment.

Pantajali's eight steps of the Yoga Sutras:
Yamas: explores truth, morals and personal conduct
Niyamas: contentment, surrender of the ego
Asanas: steady poses
Pranayama: Control of breath and vital energy; together with asanas
Pratyahara: withdrawal of the senses
Dharana: mastering concentration
Dhyana: examines meditation
Samadhi: experiencing the super-coconscious state

To learn more about the Yoga Sutras, consider the book The Yoga Sutras of Patanjali: Commentary on the Raja Yoga Sutras by Sri Swami Satchidananda.
The Physical Benefits

The focused activity of asanas is said to:
Lubricate the joints, muscles, tendons and ligaments
Improve circulation and the activity of the nervous system
Increase flexibility
Release tension

As the body is being nurtured and conditioned through regular, nonabrasive movement, yogis believe asanas expand mental faculties and enhance spiritual capacity as well.
General Guidelines to Practicing Asanas

As with any other physical motion, there are certain recommendations designed to enhance results. For example:
Stay hydrated throughout the day. Have at least eight ounces of water prior to practice.
Perform asanas on an empty stomach, usually in the morning or three hours after your last meal. If you need energy, have a piece of fruit one hour prior to exercising.
Inhale and exhale through the nose unless otherwise instructed.
To aid in concentration, exercise in a calm, warm environment.
Although there is a difference of opinion on this issue, it is commonly suggested that women avoid doing inverted poses during menstruation. Other asanas may certainly be practiced during this time. However, pregnant women should definitely avoid inverted poses and certain stomach poses after the first three months.