Sperm is made in the man’s testicles. The sperm then travels from the testicle through a tube called the vas into the body where it enters the prostate gland. In the prostate, the semen is made and here the sperm mixes with the semen. The prostate is connected to the channel in the penis and hence the sperm and semen are ejaculated.
In a vasectomy, the vas or tube is blocked so that sperm cannot reach the prostate to mix with the semen. Without sperm in the semen a man cannot make his partner pregnant.
Since vasectomy simply interrupts the delivery of the sperm, it does not change the hormonal function of the testis and sexual drive and ability remain intact. Since most of the semen is composed of fluid from the prostate, the semen will look the same.
Vasectomy is thought to be free of known long term side effects, and is considered to be the safest and most reliable method of permanent male sterilization.