The prostate is part of a man's reproductive system. It's an organ located in front of the rectum and under bladder. The prostate surrounds the urethra, the tube through which urine flows.
A healthy prostate is about the size of a walnut. If the prostate grows too large, it squeezes the urethra. This may slow or stop the flow of urine from the bladder to the penis.
The prostate is a gland. It makes part of the seminal fluid. During ejaculation, the seminal fluid helps carry sperm out of the man's body as part of semen.
Male hormones (androgens) make the prostate grow. The testicles are the main source of male hormones, including testosterone. The adrenal gland also makes testosterone, but in small amounts.
Sources: National Cancer Institute, American Cancer Society