From The Case for Natural Building on their site:

Natural building is any building system which places the highest value on social and environmental sustainability. It assumes the need to minimize the environmental impact of our housing and other building needs while providing healthy, beautiful, comfortable and spiritually-uplifting homes for everyone. Natural builders emphasize simple, easy-to-learn techniques based on locally-available, renewable resources. These systems rely heavily on human labor and creativity instead of on capital, high technology and specialized skills.

If you’re not familiar with the beauty of natural building yet, check out the slide show on the NBN site for some most excellent inspiration.

Link: Natural Building Network

You can find more about Earthships at Earthship Biotecture, a company led by Michael Reynolds, generally credited with the concept of the modern earthships.