Land Art was a term coined by the artist Robert Smithson (specifically with his piece Spiral Jetty), and describes a form of art in which the art is deeply linked with the environment surrounding it. Land Art uses natural materials in order to create art, often with a particular message or significance.
Recently it has really taken off in the contemporary art scene, as it has become associated with the preservation and representation of the Land it inhabits. It has manifested itself in a number of forms, from man made objects in site specific installations, to the natural and ephemeral works of Land Artists such as Andy Goldsworthy.
Many of these artists are actively trying to draw attention to pressing environmental issues from broad events such as global warming, to more specific things like habitat loss. Overall, Land Art (and associated movements) are aimed at bettering the world through art.
Some of the Land Artists I have been inspired by are Jean Claude and Christo, Andy Goldsworthy, Nils Udo, Richard Long, James Turrell, and Robert Smithson.
Be sure to check out one of these linked artists as art is easier to show than to explain.