So glad you asked!
Arkham Horror is a board/role-playing game based on the stories and mythos of H.P. Lovecraft. Basically, it starts like any other board game. Everyone picks a character and places their game piece on the board. They are allowed certain spaces for movement around the board during their turn. Cards are also drawn and whatever is written on the cards describes what is happening to the "investigators" during their turn. Dice are rolled to determine how the characters react to the situations presented by the cards.
Given that the game is based on a horror fiction writer's warped imagination, you can imagine that some of the things that can happen to your little game piece guy can get pretty weird. For instance, it's not unusual to get sucked into another dimension, be forced to "fight" horrible monsters, or go insane. In fact, going insane is a major element of the game. The final goal, which all players share and work cooperatievly to achieve, is the prevention of the waking up of the "Old One" or, heaven forbid, his/her destruction upon awakening.
If it sounds complicated, just wait...there's MORE!
This game starts with one board and oh...a hundred or so cards. There are a whole bunch of optional players, a whole bunch of optional Old Ones, and a plethora of different monsters to keep track of. Two of the expansions add game boards with additional locations, and all of the expansions add more cards, monsters, player characters and Old Ones. As of now, my DH has devised a special holding system such that all of our expansion materials can fit into 2 large game boxes, and even then, it's tight. In other words, the game is NEVER the same twice. Not even close.
But wait...there's STILL more.
If that's not enough of a challenge for you, the game creators have extra monsters Old Ones and tough scenarios on their website. Also, truly geeky persons (cough...DH) have even made their own mini-games. For instance, we now have a Christmas themed scenario with evil Santa and his reindeer minions.
So, if you ever wondered: How do you keep 4-8 people with advanced degrees and waaaay too much time on their hands busy for 6 hours straight?
Your question has been answered.
http://www.fantasyflightgames.com/edge_minisite_sec.asp?eidm=6&esem=1