The prostate gland is part of the male reproductive system and the male urinary system. The prostate gland sits below the bladder and when a man urinates the urine goes through the center of the prostate or the prostatic urethra.
From a reproductive point of view, the prostate adds secretions to the semen as it comes up from the testicles. Behind the prostate is a pair of organs called the seminal vesicles that also contribute secretions to the ejaculate.
I have found this video from Australian Prostate Cancer Research to be a very helpful in understanding the function and anatomy of the prostate glad.