Used widely in the science of ecology, the quadrat is simply a frame of standardized area. Quadrats are usually (but not always) square and made from PVC. They can come in all sizes, and may or may not be gridded into smaller areas. Quadrats enable the quantification of stuff in nature in a standardized, repeatable manner, and that is the key to their utility. Note that a quadrat is not to be confused with a quadrant, which refers to a quarter of some area, and is an entirely different thing.