In some research designs, the researcher will be interested in sampling individuals from the community in general, rather from some particular subset of individuals with special traits or characteristics. When a researcher samples from the community in general, this is called a community sample.
For example, some studies of the prevalence of borderline personality disorder (BPD) draw their data from treatment samples (i.e., from a collection of individuals who are seeking psychiatric treatment). Other studies, however, draw from community samples -- in these studies, people are selected at random from the community, and the presence of BPD is assessed. Rates of BPD are much higher in treatment samples than in community samples.
It is important to pay attention to whether a particular study draws from a treatment or community sample because this will affect the findings.