Tickets went on sale July 31st for Mickey's Not-So-Scary Halloween Party at Disneyland. It's a special Halloween-themed event held on various nights through September and October. It requires its own admission ticket, which runs from $62-$77, depending on the day of the week and proximity to October 31st. The Halloween stuff starts after 7pm, when the park ushers out general admission visitors. Last year Halloween attendees were allowed to enter the park early, at 4pm -- hopefully that remains true this year as well, in which case you get a pretty good deal for 8 hours at the park. Last year they also had many trick-or-treat stations where candy was dispensed indiscriminately; I scored a good amount of common "fun-sized" candy without a costume (or maybe I'm naturally scary-looking).
As far as what actually happens, a lot of areas are Halloween themed, in the same way the Haunted Mansion gets its annual Christmas makeover. A highlight of my visit last year was the undead barbershop quartet singing on a raft in the fog on that "river" in New Orleans Square. As I recall all the rides were open as normal, as well as most shops, and some buildings were set up as trick-or-treat stations. There was also a fireworks show and a parade that involved characters like Donald and Goofy dressed in Halloween costumes.
If you're interested, tickets tend to go fast. Also look into the dates--including most Thursdays, Fridays and Sundays--if you're planning to make a trip sometime during September or October, since you'll have to leave at 7pm if you don't have a Halloween ticket.