In layman terms it is a study that investigates interventions in human volunteers and assesses the health outcomes. It is different from other studies, as the human is the subject of the study. Interventions are performed, that include medicinal products, medical devices, procedures, behavioral treatments, preventive care or diets.
what is a Clinical trials are interventional studies, that are different to observational studies, where no interventions are performed. For objectivity reasons, many clinical trials compare new interventions with already available interventions or a placebo.
When an unknown intervention is studied, the safety, efficacy and potential helpfulness is measured on the outcome in the study participants. For example, a new pain medication can be assessed in a clinical study for its effectiveness in decreasing pain.
New interventions require clinical trials to test safety, side effects, adverse effects and efficacy. Therefore, there can be up to 4 different phases of clinical trials.