Tumor markers are substances that are released by cancer cells or produced by the body in reaction to a tumor that is present. Normal cells also make these substances, but they are produced in much greater amounts by cancer cells.
Tumor markers are usually measured in the blood or the urine.
The most common use of these markers is to follow a known cancer. In this setting a decrease in the level of a tumor marker may be a sign that a tumor is decreasing in size, whereas an increase in the level could mean a tumor is progressing. Tumor markers aren't usually used alone, but rather in combination with a physical exam by your physician and other studies such as CT scans.
Sometimes tumor markers are used to screen for cancer as well. An example of this is checking serum PSA levels as a screen for prostate cancer.