Yep, the name says it all! ;D
Pokemontcg.com or Pokemon TCG online is where people can battle in the trading card game.
Here, you can personalize your TCG deck, enter in codes that come with the packs/theme decks that you buy, and make some friends! And the best part is, you don't have to pay for anything as opposed to the actual TCG which had so many variables that it would take forever to list them all!
But in short, Here you play the TCG online against either the computer or another person. You customize your decks and receive additional virtual cards after buying the real cards. So, have fun on Pokemontcg.com, and make a killer deck! :D