Simplifying geometry drawn in a distance is a common technique for lowering the processing power needed to render the whole scene. The simplest geometry that can be drawn is a flat quad (or a triangle) with an image depicting the look of an object. For the effect to be convincing, the quad must always be facing camera:

Flat objects that are always facing camera are called billboards. They are used not only for distant objects as the lowest level of detail of a geometry, but also for particle effects.
One straightforward technique for drawing billboards is to use geometry shaders.