It is literally the trading card game - online. Does not reflect what the state of the game was when it launched as I did not play it at launch. Note that this is a review is of this time I'm posting it. The other cool thing is if your a pokemon card collector and still buy pokemon cards, the pokemon packs come with a code you can put into the game so you can use in the online game. They have event which act like mini tournaments giving you a chance to win even more rewards. You earn coins in the game that you can use in the shop to buy booster packs and theme decks. There's a single player where you play against the AI and can earn booster packs or play against real people and earn rewards there as well. Other than that the game is great and completely free which is what makes the game so much better. Matchmaking pairs you up with anybody so if your new to the game you might end up playing a long time player who will have better cards.
The only bad thing would be it's matchmaking. Of course because it's online it has a tiny bit more regulations but it's only to help keep the game balance. It's pretty much the card game but online.
If you like playing pokemon cards then you'll love pokemon tcg online.