The deck-building game of Ascension has a number of expansions. The least of these are the 'theme packs', small sets of 10-12 cards that can be mixed into the game to modify game play. The Rat King and Rat Queen packs were the first two released, and as a result are two of the cheapest. They are also, arguably, some of the best of the theme packs.
Each pack comes with two members of royalty (either two rat kings or two rat queens), and ten giant rats. The two members of royalty are shuffled into the deck, and act as standard monster cards -- except that they also trigger a rat swarm.
The only difference between the K and the Queen is that swarms have a slightly different effects, and it is simple to house rule them to be equal. However, going strictly by card text, when the Rat King comes out a rat swarms over each other card in the center row, and that card cannot be purchased or defeated until the rat covering it has been defeated. The Rat Queen is exactly the same, except that the cards in the center row are first shuffled face down, then covered, meaning that when you defeat a rat minion you do not know which card you might be uncovering.
Both the Queen and King require six attack to defeat, and both have the effect that when they are defeated all Giant Rats (one attack to kill, one VP reward) are automatically defeated. This has the effect that killing a Rat Royalty early on gives you a lot of victory points, but after only a few rounds of picking off the minions they become fairly useless (the reward for killing a Rat Royalty with no remaining followers is only 4 VP, and thus usually not worth it if other monsters are present).
Both the King and Queen sell for $5 on the Ascension website, and might be found slightly cheaper elsewhere. Most people do not purchase them, or purchase them only incidentally in the Year One Collector's Edition, which includes both the Rat King and the Queen sets, plus the Leprechaun Pack. While they are often fun to have in play, there is no need for both packs unless you are a collector.