This 3D animation from The Human Atlas explains Prostate Removal. 

The prostate is part of the male reproductive system. It is a small, round gland that is located in front of the rectum at the base of the bladder. Its primary function is to release fluid into the urethra during ejaculation.

During ejaculation, sperm travels from the testicle through tubes called the vas deferens. The vas deferens run behind the bladder and enter into the prostate gland. During its journey, sperm combines with seminal fluids, other components of ejaculate, from three sources - the seminal vesicles, the prostate, and the bulbourethral glands. The combined fluids, called semen or ejaculate, then travel the length of the urethra and out of the body via the penis.

If prostate cancer develops and is not detected early, then the prostate may need to be removed.  There are several surgical approaches to prostate removal. Looking inside the pelvis, the anatomy around the prostate consists of many delicate structures including muscles and nerves that affect both urination and erection.  During the removal of the cancerous prostate, the surgeon must use precise instruments to spare these important structures.

The urethra, or urine channel, is carefully cut at the base of the bladder. The added control and accuracy of the instruments may also assist the surgeon in preserving the nerve bundles responsible for erection.  Once the prostate is cleanly detached, the prostate is then removed through a small incision.

A catheter is inserted in order to drain the bladder after surgery. The bladder is then re-attached to the urine channel over the catheter.  This catheter is left in place for a short period of time to ensure proper healing.