Head lice are small insects that live in the scalp by periodically biting and sucking blood.
The mature adult lice are able to attach the eggs, called nits, to the hair shaft with a strong gluey substance. This can make it quite difficult to remove the nits even after they have been successfully killed. The bites themselves are not what make head lice so bothersome for the person who has them. It is the itching. The itching is caused by the reaction the person has to the chemical the bug injects into the skin when it bites.