Scientists use a variety of sampling techniques in order to get an estimate of the number or quantity of different species in an area. One of the tools they use is a quadrat. A quadrat is a square with a known surface area that is sometimes divided into smaller parts. By counting the number of each type of species in the quadrat scientists can the estimate the number of these species in the entire area.
Random or systematic sampling is important when choosing where to place the quadrat. If you make a choice about where to sample you could be biased and not get a good number of estimating because of your bias. There are many ways to random sample, including mapping the area to a grid and using a random number generator, measuring a set distance and recording data a set points along the measured line, tossing a ball over your shoulder, or any combination of these. 
Quadrat  from this website.
