Asana is a Sanskrit word that literally means a seat but in the practise of yoga refers to a pose or posture. In Patanjali's yoga sutras Asana means, mainly, sitting for meditation.

The practice of asanas involves stretching and moving the body into various positions. With practice, the body can be made to remain a given position for a longer period of time, comfortably. When a degree of comfort is attained in a given posture, it becomes Asana. In general, however, the term is also used to refer to physical yoga exercises in general.

The practice of asanas is very good for muscle flexibility, and many practitioners believe the positions massage and bring balance to the various internal glands and organs of the body, and that they facilitate and balance the flow of prana (vital energy) in the body.

This physical aspect of yoga has been much popularised in the western world and is practiced by a great deal of people, including many celebrities like Madonna, which has given rise to a misconception that asanas are all there is to yoga. This is not true. Yoga asanas such as shirsasana (headstand), etc. (see below) are actually part of Hatha Yoga, which itself is just one of several different ways of practising and approaching yoga. For example, in Pantanjali's work referred to above, Asana is classified as the third rung in the ladder of the practice of Raja Yoga, which consists of 8 limbs: Yama and Niyama, which are ethical obligations, Asana, Pranayama, which is breath control, Pratyahara, which is sense withdrawal, Dharana, which is concentration, Dhyana, which is meditation, and Samadhi, which is the experience of unity with God.

Conditions for a Good Asana
The Asana should be firm and easy. It should be steady and not cause discomfort of any kind. Any tightness or tension observed in the body should be consciously relaxed. It should be a comfortable posture in which he or she can sit for a long time.

The Asana should be effortless both in the body and in the mind. Absolute ease of relaxation is the sign of perfected Asana. The breathing should be a natural rhythm, through the nose, breathing into the belly and not into the chest.

According to Hatha Yoga practicioners, when this bodily control is achieved, they are free from what they call the 'pairs of opposites', such as heat and cold, hunger and thirst, joy and grief, and so on.

Selected Asanas
There are hundreds, if not thousands, of yoga asanas ( since in reality, any bodily position comfortably held could be an Asana ). There is a wealth of knowledge in books and on the internet, but it is better to begin practicing with an experienced yoga instructor, who can see if you are performing the position correctly, and can prevent you from hurting yourself by overstretching. These days, it is not difficult to find yoga instructors, although good ones are perhaps harder to come by.

The Hatha Yoga Pradipika By Swami Svatmarama suggests the following "sitting for meditation asanas".
Other translations of the Hatha Yoga Pradipika have more to say about the sitting asanas, albeit cryptically.

* Svastikasana: "Sitting straight on level ground, squeeze both feet between calves and thighs [of the opposite legs]. This is svastikasana."
(IMG:http://www.sacred-magick.com/img/Svastikasana.gif)

* Virasana: "Place one foot upon the other thigh and the other foot below the opposite thigh. This is virasana."
(IMG:http://www.sacred-magick.com/img/Virasana.gif)

* Siddhasana: "Press one heel into the place below the sex organs [the perineum] and put the other heel just above this region [close to the abdomen]. Press the chin upon the chest, sit up straight, with controlled organs, and fasten the eyes between the eyebrows. This is siddhasana, whereby all obstacles on the path to perfection are removed"
(IMG:http://www.sacred-magick.com/img/siddha.jpg)

* Vajrasana: "Place the right heel above the sex organ and the left heel over the right. This too is siddhasana."
(IMG:http://www.sacred-magick.com/img/vajra.jpg)

"Some call this siddhasana; others say it is vajrasana, or muk-tasana, or guptasana"

*Padmasana: "Place the right heel upon the base of the left thigh and the left upon the right thigh. Cross the arms behind the back and grasp the toes, the right ones with the right hand and the left with the left. Press the chin on the breast and look at the tip of your nose. This is called padmasana and cures all diseases."
(IMG:http://www.sacred-magick.com/img/padma.jpg)

~ Thanks to mystic for the following links for more complete and detailed asana:
Yoga Practices - Positions, Postures, Asana
Yoga Posture Index - Sorted by Sanskrit Name

An easy way to view other pictures is to go to http://images.google.com/ and search for the asana you're looking for