A clinical trial is a careful investigation to determine if a potential therapy has intended effects for patients in the treatment of a disease and if the therapy is safe.
Early stage, or Phase 1, clinical trials are conducted in a small number of people and focus primarily on safety.
Phase 2 trials typically follow a Phase 1 trial, involve a larger group of people and measure how well a therapy might work in addition to providing more information about safety.
Phase 3 trials are conducted often in hundreds of patients and are often designed to provide enough information for regulatory approval and broad availability of a new therapy.
More information about clinical trials can be found at the National Institutes of Health (NIH) website, here.