Hi Fiona
There are the Hallowe'en Horror Nights in Universal and Mickey's Not So Scary Hallowe'en party in the Magic Kingdom, here is a link to the info we have - the dates for this year have not been announced yet.
http://www.wdwinfo.com/holidays/halloween.htm
Here is a description of Universal's HHN - ".......... They have hundreds of scareactors all across the park - in the streets, on rides, and in the five haunted mazes. The lights in the park are left off, or replaced with black lights and strobe lights. All the streets are covered in thick fog. The event is very intense. Scareactors will jump out at you, shake cans of rocks in your face, and chase after you the entire night. It is a very popular event, and on weekends especially, lines will be long."
Whilst Disney's
MNSSHP as it is known, is a very gentle affair much more suitable for younger children and also lots of people dress up for the occasion. You cannot dress up for Universal's HHN since the actors need to be distinguishable from the public.
Both require separate admission, but if you are in Universal you can upgrade your park admission to a "Stay & Scream" !!