A vasectomy is a more or less permanent form of birth control for males.
It involves making a small incision in the ... ahem ... scrotum and cutting and tying the vas deferens so that sperm cannot pass through to the prostate gland - and therefore none go out the penis via the urethra during sexual intercourse.
Reversals of vasectomies have been done successfully, but the success rate of these reversal operations is miserable. If you are thinking of a vasectomy, expect it to be permanent.
The complication rate of this minor operation is very, very low in the hands of an experienced operator. It can be done under local anaesthetic.