Like with nearly all free online gaming, the revenue model here is in microtransactions that sell you piecemeal items that enhance the experience in some way. In this case, it's cards. You only get 32 cards (eight from each of the four factions) in this free version, so if you want to beef up your offense you should expect to plunk down some scratch in digital form. EA has a FAQ up if you want the full rundown of how this free version works.
Usually when I see the phrase "card-battling" I turn and head the other direction, but BattleForge is actually pretty cool. Since your units correspond to cards, the deck you build determines what units you can produce in each match, so there's a ton of possible combinations. It's like having four different factions in a typical RTS, and being able to summon units from all of those factions in a single game. There's also no base-building and you can spawn units anywhere on the map that you already have guys hanging out, so the game seems to focus mainly on combat, from what I've played.
Head over to the official site to grab the game client; you can't get past the front page without getting whapped in the face with a download link.