Study arms

In a clinical study, participants are often divided into groups.

In interventional studies, participants are prospectively assigned  to arms and receive different interventions. Assignment to an arm  is typically random. Multiple arms allow the effects of different  interventions to be compared. Participants in a control arm may  receive no intervention, placebo, or an intervention with a known effect.

In observational studies, a pre-defined population may be observed  over time and is termed, a cohort. A cohort consists of individuals  with some common characteristic(s), for example, age, place of   residence/employment, or medical condition. A cohort is observed  over time to determine if the presence or absence of an exposure is   associated with a health outcome or condition. Other observational  studies may compare those with a condition (cases) to those without   a condition (control) and evaluate differences in exposures,   treatments, or behaviors. The cases and controls may be selected  from a larger population or a cohort.

Source
Clinical Trials.gov