Prostate specific antigen (PSA) is a protein produced by the cells of the prostate gland. 

Because PSA is produced by the body and can be used to detect disease, it is sometimes called a biological marker or tumor marker. It is normal for men to have low levels of PSA in their blood; however, prostate cancer or other conditions like benign prostatic hyperplasia and prostatitis (inflammation of the prostate) can increase PSA levels.

References

National Cancer Institute: http://www.cancer.gov/cancertopics/factsheet/Detection/PSA