I do not know if Dominion is the innovator of the deck building genre, but it was definitely the game that put the deck building genre on the map. Since it came out, many others have attempted to take the concept and make it their own (Thunderstone, Resident Evil Resident Evil Deck Building Game, Ascension Chronicle of the Godslayer, Heroes of Graxia), but none of them have come anywhere near the success of Dominion.
In Dominion, you select 10 random "kingdom cards" (out of the 25 that come with the game), and these cards along with some standard cards such as money and victory points are what you have available to construct your deck. From here, on every turn you are able to play an action card (generally one of the 10 kingdom cards) and then buy a new card to put in your deck, so that you are actually creating your deck as you play the game.
With that said, Dominion has several things that make it a fantastic game. First, replayability is off the charts. With 25 different kingdom cards in the base set (and a ton of expansions!), you can play the game hundreds of times without having quite the same mix. Another key feature is that the game is very simple to learn - you simply play an action, buy a card, and end your turn. When Rio Grande made Dominion, they also put quite a bit of thought into the packaging, and so the game comes with essentially a mini-filing cabinet (which is even labeled) that allows you to quickly locate the kingdom cards that you choose to use in any given game.
About the only negative about the game is that there are some kingdom cards that are almost completely worthless (the Chancellor is an example), but this isn't all bad, because it could also challenge the players to make a good deck with horrible cards.
Overall, I give Dominion a 9.5/10. It is, currently, the undisputed king of the deck building genre. If you have ever played a Customizable Card Game (like Magic: The Gathering, Star Wars:CCG, Star Trek:CCG, Yu-Gi-Oh, Pokemon, etc), I would highly encourage you to play this game. It has a lot of the same kinds of strategies that are found in those games but without the constant need to buy new packs!
Want a second opinion? Try checking out Play Board Game's Dominion Review, or I Slay the Dragon's review of Dominion.