Yeah from what ive always known, the idea for Halloween is that you take kids (ages 5-15, ish) and take them door to door from 530 - 730 in the evening on Halloween night, to houses with the porch light on. Ring the bell, say "Trick or Treat!" and the person gives you candy.
The parties come from kids who have grown up and miss an excuse to dress up, but because the group is more 16-25+ crowd, girls wear less and less clothing and drink more and more.