Pokémon Trading Card Game Online is a browser based Battle Cards MMO Game.
Much like the trading card game it's based on, Pokemon TCG Online allows gamers to collect cards, build custom decks, and challenge other players. It's absolutely free of charge to register and play, and you're even provided with three complete decks to play with if you don't want to spend money on booster packs. Playing through games (whether you win or lose), or even just completing the tutorial, also earns you tokens, which can be used in the game or on the main Pokemon site. There are also achievements that can be unlocked, such as one for knocking out 1,000 Pokemon, one for confusing five monsters, and so on.
One of the best parts about Pokemon TCG Online is that it's easily accessible to anyone -- from TCG noobs to hardcore enthusiasts. Another great feature about this online experience is that you can play either against the computer or against other players who are online at the same time.
The only real issues that arise from playing the game is the fact that the login IDs avatar is not the same as the in-game avatar, what are you waiting for, just try it.