Two tubes (called vas) in the scrotum are cut and tied off. That stops sperm getting into the semen (cum). The sperm are reabsorbed by the body. It's a 15-minute operation and you walk out of there....
Some types are done by removing a section of the vas deferens which carries the spermatozoa to the urethra. Other methods involve tying them off. In either case, it is very difficult to reverse. Soon...