When abnormal cells divide uncontrolled, that it causes called Cancer. Some cancers may eventually spread into other tissues. Cancer may be start almost anywhere in the human body, Normally, human cells grow and divide to form new cells as the body needs them. When cells grow old or become damaged, they die, and new cells take their place.
When cancer develops, however, this orderly process breaks down. As cells become more and more abnormal, old or damaged cells survive when they should die, and new cells form when they are not needed. These extra cells can divide without stopping and may form growths called tumours.

Common Symptoms of Cancer
Symptom or Sign Action to be taken 
Change in bowel or bladder habits   Ultrasonography and endoscopy  
A sore that does not heal  Biopsy and oral and skin examination
Unusual bleeding or discharge in stool   Rectal exam and colonoscopy
Unusual bleeding in between periods  Gynaec examination for cervix and biopsy
Thickening or lump in the breast, testicles, elsewhere  Ultrasonography and FNAC if abnormal
Indigestion of difficulty swallowing  Endoscopy
Obvious change in the size of mole, or mou sore  Biopsy
Nagging cough or hoarseness  ENT Examination and X-ray chest