We'll start with the ratings:
Expected Length of Game Play: 30 minutes to an hour
The idea behind this game is that you're a lord of a medieval or fantasy kingdom, trying to build up your influence and power such that you have a greater realm than the other players. All of this is accomplished through the various cards.
Before I describe the cards, I want to point out what I thought was an ingenious reversal of the normal arrangement of game play. With most card games of this type, you start with all the cards shuffled together into one or two decks, possibly three or four, but from these decks, the players draw their cards and sort them into the various types. Dominion is the opposite.
Game starts with each card in its own stack (so, for example, all the 'workshop' cards are in one stack whilst all the 'militia' cards are in another stack, and so forth). Through the course of the game, players purchase these cards and shuffle them together to create their own deck.
Here's what happens. There are three categories of cards: action, treasure, and victory points. You start the game with seven bronze treasure cards (each worth one treasure point) and three estate cards (each worth one victory point). These ten cards form your deck, from which you draw five cards to form your hand. The victory points don't do much for you at the beginning of the game, but you'll want to start buying them eventually.
In the meantime, the treasure cards are used to purchase action cards, larger treasure cards, or victory points cards. On your turn, you get to play one action card (certain cards allow you to gain extra actions in a turn). Then you get to purchase a single card (some action cards increase the amount of cards you can purchase in that turn) of any type. Then you discard all the cards remaining in your hand and draw five new cards.
Now before you think, 'That's a waste of cards,' you must realise that when you exhaust your deck, you reshuffle your discard pile and start drawing from this new deck. So when you're spending a treasure card to buy, for example, a larger treasure card, you're only discarding that treasure card for the time it takes to get through your deck again. Thus, what starts out as a small deck of only ten cards is actually allowing you to slowly increase the number and variety of cards in that deck. So the game basically boils down to using the cards in the deck to add more cards to that deck in the most efficient manner possible.
Once three different stacks of cards are exhausted, or the 'province' cards (the victory point cards worth 6 victory points) are exhausted, the game ends, and whichever player has the most victory points in his deck is declared the winner.
The game is fun, in a planning-for-the-future sort of way. In my opinion, the only problem with the game is that there's not a lot of interaction between the players. In the basic game (which is the version I played), the only time the actions of one player affect any other player is if someone plays a militia card, which forces all other players to discard two cards. This means that when it's not your turn, there's not much reason to pay attention to what other players are doing. But that can be an advantage; the game is great for playing whilst the telly is on, for example. The player who's turn it is can do his thing whilst everyone else watches TV, and when he's done, he indicates to the next player that it's his turn. The other player needn't worry about having missed anything.