A tool for assessing biodiversity using a confined area or subunit of a larger area. Usually, a square frame (can be other shapes) which is used to subsample in an area usually for plant diversity, although it can be used for many other organism-groups.
The quadrat is placed on the ground at the sample site, the size of the quadrat, the number of times it is used, and whether the sampling was performed systematically or randomly are all recorded. All this gives information that can be used to extrapolate diversity in an area. The information can also be used to find the cover of a specific (or several) organism(s) etc.
When choosing the sampling method, it is always a good idea to keep in mind resources available. Infinite resources allow for a more thorough approach. The sampling can then be done in a systematic way, this is preferred.
With limited resources, another approach is needed. To sample a large area with a truly random placement of the quadrats is a far more pragmatic approach, and will give an approximation of the systematic results.
To avoid study-bias a preplanned sampling method is preferred, over one directed by the appearance of the sample site.
When choosing the number of quadrats needed to study an area. Consider what types of organisms are being studied, and how large an area you are sampling from. More is better, but in most studies, there is not enough time or resources to sample everything. Consider the reason for the study, and how frequently organisms reoccur, and how often "new" organisms are found. Another thing to consider is the density of organisms in an area, if densities are really low getting more sample-points are vital.