The premise is simple. Lay out a grid of single words, give your partners a single word clue that will lead them to chose the specific words you want them to pick out of the grid - while avoiding words you don't want them to pick. Sometimes, what's obvious to you, isn't so obvious to those not inside your head!
A group of 6 of us played 5 games in just over an hour, with two Spymasters and two sets of two teams. It was a lot of fun, easy to learn, hard to master.
The materials are a little cheap, but I suppose it keeps the price down and the cards could likely be covered easily with the smaller card sleeves.