There are different types of clinical trials:
A treatment trial tests new treatments, such as a medication or a surgical technique.
Prevention trials look for better ways to keep people from getting a specific disease.
Diagnostic trials are used to find better tests or procedures to make a diagnosis.
Screening trials test the best detection methods.
Quality-of-life trials are focused on comfort and life issues of those with a chronic condition.
Source: The National Institutes of Health.