The endpoint in a clinical trial is an event such as the occurrence of a disease, or symptom, or a particular laboratory result. Once someone reaches the endpoint, they are generally excluded from further research in the trial.
A surrogate endpoint (or marker) is a measure which in itself is not the outcome that the study treatment aims to elicit. For example, blood pressure is used as a surrogate endpoint in trials because it is a risk factor for heart attacks and strokes – even though in itself blood pressure might not be important for patients.
Surrogate endpoints are useful if it would take a very long time for clinical endpoints to appear. Surrogate endpoints must be proven to be valid markers of clinical endpoints when they are used in clinical trials.