Cancer screening is looking for cancer before a person has any symptoms.  Some cancers can be found early before they have a chance to grow and spread. Some pre-cancerous conditions can be found and removed, preventing cancer from occurring.  Depending on your individual cancer risk and family history, your doctor may have personalized recommendations for how often you should be screened and what types of screenings should be completed. 
Screening tests are not meant to diagnose cancer.  When a screening test is abnormal, more tests, like a biopsy, may be done to diagnose cancer.