Halloween Horror Nights is a hard ticket event,meaning it's separate from regular day admission.The Universal park closes at 5pm on HHN days and regular day guests are ushered to the park exit as the street props are added and scareactors get ready to go to work in the houses.They also have what they call the "Stay and Scream area" which is usually in the street where Finnegans Pub is located.If you have regular day admission and HHN tickets you can go in here in the time the park is getting transformed for HHN.Being in the Stay and Scream area allows you to get a head start on the regular crowds waiting outside the park as they'll usually allow you to head to the 2-3 houses that will open around 6pm which is before the official opening time and before the regular crowds come into the park.If you add the HHN tickets to your day ticket or AP you will get a significant discount on regular HHN admission.Some rides will be open during HHN but remember this is a very popular event that fills up with lines reaching as much as 2 hours on weekend nights so if your main goal is to do the houses and HHN shows doing the rides will lessen your chance to get all the houses in,unless you purchase the express pass which is pretty expensive.Having an AP I never ride the rides,always stay in the Stay and Scream area and only do non peak nights,doing this I never have to purchase the express pass and always get to see all the houses and shows.
HHN requires a separate ticket for admission as Doom stated. There are usually four or five attractions open during HHN but most of us don't fool with the rides since you can do those during the day. We concentrate on the houses and festivities of HHN since it only lasts a month.