Moshi Monsters is an online game world for kids aimed at a marginally younger audience than Neopets. Neopets attempted a TCG (published by WotC starting in 2003, discontinued in 2006), so naturally Moshi Monsters is attempting to do the same with a TCG called Moshi Monsters Mash Up, published by Topps.

The UK and US decks are slightly different (the US packs have an extra card and cost a little more).

The game plays exactly like Top Trumps; each card has two values, and you pick one to compare to the value of your opponent’s card. Unfortunately, being a collectible card game, you can all (assuming you have the money) stack your decks with better and better cards, until everyone is playing with identical decks.

(official site)