The previous reviewer summarized the game nicely. It's a big box with literally hundreds (700+) pieces if you count them individually. The rules are even designed for solo play if you can't find a friend or two (and this happens very frequently if you look at Final Fantasy's forums). Top quality production value. You definitely see where the money went. The only drawback, and thus only 4 stars, is the amount of time it takes to play a game. Give yourself at least 4 hours. More if you're new.

The Arkham Horror is a game that plays on several levels; for the Cthulhu mythos fans, there are eight different ‘Ancient One’ threats from outside and minions that include the hounds of Tindalos, the mi-go, the ubiquitous cultists of the Cthulhu mythos (whose combat abilities vary, according to precisely which ancient one threatens Arkham in any particular game) and one of the terrifying dholes; for those who like to try to plan their games, there are resource options to manage and decisions to make which threats to go after- and how soon; and for those who just like weird things happening there are a selection of random encounters for each of the locations in Arkham, or for other worlds.
Confrontations with the ancient ones, their minions, or other challenges are usually resolved through a system of rolling six sided dice, in an effort to score sufficient successes to better the opposition. Various items, spells, allies, or skill training can raise the number of dice that an investigator rolls for a check, as can the expenditure of valuable clue tokens. And for those investigators for whom the horrors prove too much, there remain options such as the local hospital or asylum to recuperate.
With other worlds to explore, monsters to fight, gates to shut, and (if everything goes badly wrong) a final confrontation with an Ancient One in a last ditch effort to try to 'save the day', The Arkham Horror represents a 4-star slice of Mythos flavoured fun in this reviewer’s opinion; there are several excellent expansions also currently available, which increase the variety of options open and the threats faced, making a good board game great.