What is it?
Rammed earth construction is a building technique which utilizes soil for its primary building material. It is known variously as pisé de terre or simply pisé. In many ways it is a precursor to modern concrete.
In a rammed earth construction, the structural components (walls, i.e.) of a building are built of moist (but not wet) soil, mechanically compacted into a mold. In this manner, the soil will end up with most of the characteristics of natural sandstone.1 The soil dries fairly quickly (within hours) enough such that the forms can be removed. Within an hour it will have dried and hardened enough that texturing the outer surfaces by brushing will be impossible. The wall will continue to harden as it cures over the next several months to years, but after a few hours it should have a compression strength approximately one-quarter to one-fifth that of cured concrete. In other words, it will be able to withstand pressures per square inch at the top of the wall approximately one-quarter to one-fifth that of an equivalently-sized concrete wall. New Mexico code, for example, expects and requires a minimum strength of 300 psi for a rammed earth building wall. Given that a rammed earth wall is generally much heavier than a concrete wall, care must be taken in sizing the foundation properly!
The composition of the soil used will make a difference; too much sand and the wall won't bind, and too much moisture and it won't compact properly as the water content resists pressure, or the water pools inside the wall. The strength can be increased (both compression strength and resistance to exterior damage) by adding cement to the soil before compaction. This still won't result in proper concrete but will increase performance. Just like concrete, a rammed earth wall can be internally braced with rebar or fiber in order to resist deformation or cracking.
Why use it?
A rammed earth wall will offer resistance to water. Properly hardened it will resist rain, but it is vulnerable to long-term erosion. As a result rammed earth walls should be protected from floodwaters by being placed on high ground with a high foundation lip and from rain flows by wide verandas or eaves to allow rainwater to flow away from the walls.2 If such a wall does become damaged, it can be easily repaired by adding a soil mix to the damaged area and compacting it into the wall. If the original soil mix is handy, the repair will be very difficult to see.
Compared to concrete or wood-frame construction, a rammed earth wall is cheap in materials if somewhat more expensive in labor. This is important for use in poorer areas, or for homebuilders working on their own homes. Especially if the local soil is suitable, it may be much cheaper to use this technique than to transport other building materials to the site, especially if wood is at a premium such as in plains or even desert areas.
In areas with high temperature differentials over the day (again, like a desert) a rammed earth structure will serve as a heat sink. During the day it will absorb warmth without transmitting much of it, and when the night cools it will radiate the heat it absorbed during the day. This makes it ideal for evening out temperature cycles inside a structure without the use of (or with less use of) active heating or cooling.3 It is not a very good insulator, however, so its use without an insulating layer is not recommended for areas where the temperature is consistently cold or consistently hot (and internal heating or cooling is used). Modern rammed earth building techniques suggest insulating such walls from the foundation with a layer of styrofoam and providing interior insulation between the wall and the dwelling area.
Notes:
1: http://www.motherearthnews.com/Modern-Homesteading/1973-09-01/How-To-Build-a-Rammed-Earth-House.aspx
2: http://www.greenhomebuilding.com/QandA/rammedearthQandA.htm
3: http://www.rammedearthhomes.com/technical.htm