Crop Circles are geometric formations appearing usually in crop fields (most typically wheat and barley, but also lots of other crop). There are also sightings of similar formations in snow, ice, sand, grass, surface of the sea and even clouds. During the past years several rings have been reported in the ice covering lakes in the winter, mostly in Canada and USA. It can't be said for sure whether this is the same phenomenon or something quite different.
The term Crop Circles originates from the early circular shaped patterns in England. Even today the patterns are constructed mostly of circles and rings, but often quite complex shapes are born from the intelligent combination of circular and sometimes linear components. Single circles are also still appearing. Practically in all "classical" English patterns arcs, spirals and other such forms are made with parts of circles, not ellipses or "true" spirals - even if there are some rare exceptions. Formations with no circle elements at all are very rare.
In Crop Circles the stems are not cut but bent down. In circle-shaped portions, the stems are usually laid-down in spiral fashion, following the perimeter. Because the formations are very large, up to hundreds of meters in diameter, they are best seen from the air high above the ground. There are, however, also very small patterns, just a few meters across. In some formations the crop is tightly pressed against the ground by a physical force, in others it can be clearly seen that no strong physical pressure was applied. Indeed, in some Crop Circles the crop is not at all laid down against the ground, but instead bent higher (for example in the middle of the stem).