According to earthship.com, earthships are,

the epitome of sustainable design and construction. No part of sustainable living has been ignored in this ingenious building.”

The term was popularized (and trademarked) by Mike Reynolds, an architect who is the leading pioneer in the field. Check out some pictures of some of the coolest earthships already in existence (click an image to enlarge):

Earthships are built using 6 fundamental design principles:

All heating and cooling uses natural thermal or solar energy; Electricity is independently generated using solar or wind power, with excess stored in batteries for later; ALL water must be collected naturally (through rain, snow melt, etc.) and used 4 times; ALL water is recycled, with sewage water being treated on-site and reused as gray water for the flushing of toilets for example, and other used water (like from washing dishes and clothes) re-used for things like small-scale food production; All materials must either be naturally available locally or recycled (earth-filled tires, for example, are a popular material); Food is produced organically using recycled water with the goal of reducing or completely eliminating reliance on supermarkets

For more, check out earthship.com.