There's usually only one HOH - head of household. Traditionally, when the head of household gets the key to the HOH room, they yell WHO WANTS TO SEE MY HOH ROOM? , then all the other housemates, (even enemies) have to gleefully run up and check out the room, the family photos, goody bags etc. The HOH room is also where people beg and make deals and pacts if they think they're at risk, or if they're unfortunate to be put up for eviction.
When two folk are nominated for eviction by HOH, they have the chance to take themselves "off the block" by competing in the POV - power of veto contest. The HOH, the two nominees and three randomly drawn housemates compete in this contest.
The POV can be used to save one nominee, or none. The winner of the POV decides, (but must be careful not to displease folk and put a target on his own back) for going against what the house wants. If the veto is used then someone else replaces the nominee and is put up for eviction.
Sometimes the two people up for eviction are pawns i.e. just being used (sometimes against their will) till the POV has been played, because the HOH has a slightly better chance to get rid of his real target without failing and becoming a target him/herself next week.