Artifact will feature 280 cards and 44 heroes. Matches will require you to take down towers in two out of three lanes, each of which has its own board. The three lane structure is similar to Dota's, though obviously the details are quite different in a card game.
You can see more screenshots of the game here, including a more detailed look at what the hero and item cards look like. Artifact will not be free-to-play, but Valve’s plans on how to monetize the game are still a bit murky.