Adobe is an ancient building material which is in wide use in many desert regions of the world. It has also experienced a resurgence in popularity in areas of the world where people are concerned about environmentally sustainable architecture, because it is renewable, efficient, and naturally sourced. You can see many examples of this type of architecture across the Middle East, American Southwest, and Latin America, running the gamut from modest homes to sprawling complexes.
To make adobe, people mix sand, clay, and straw. Many people like to use dirt, which has a natural mixture of sand and clay, to make it. Once the materials are mixed, they are packed into molds or shaped by hand into bricks which are allowed to dry in the sun. After around a month of drying, the bricks can be used to build a structure which is mortared together with fresh adobe. It is important to pack the ground down well before building a structure with this material, because the weight of the adobe will cause extreme compression of the foundation which could result in structural flaws.
One of the major advantages to adobe is that it can be made with readily available material. Most people have mud and dead grass around, along with holes to mix the materials in. The other advantage is that it is extremely sturdy, with adobe structures surviving earthquakes, snow, and heavy rains with relative equanimity. For structures in hot regions of the world, this material is especially popular because it keeps buildings cool with its powerful insulating properties, and the buildings may be whitewashed to make them even cooler inside.
The term comes from Egypt, where people have been building adobe structures for thousands of years. The word (and material) slowly spread across the Middle East, eventually traveling to Spain with the Muslim Conquest. When the Spanish in turn conquered the Americas, they brought the word with them, although Native Americans had been building with this material for thousands of years already.
In hot climates, adobe is an ideal building material. It does not require timber, which tends to be scare in desert regions, and it is very easy to work with; a small crew of people can build a very efficient, pleasant house very quickly, once the bricks have been made. In the world of green design, adobe is favored because it is produced with renewable, local resources, just like cob, another popular mud-based building material.