Basically you pay $13.37 a month ($11.67 mo if you pay for 6 months) + S&H, and they send you cool Geek + Gamer items. The boxes range from pictures, stickers and cardboard toys, to action figures and PoP! Vinyl figures. They even giveaway a Mega Crate each month valued at $250. Sign up before 9PM PT on August 19th to receive August's Crate and use the code CAKE to save 12% on any subscription.
You can see a preview of prior crate items by hitting read more.
See an example of March 2013's Loot Crate below and click for my Unboxing of June's Loot Crate;