Close your eyes. Imagine you are a mighty lord commanding an army. Or a magician summoning creatures onto a battlefield. Or a farmer who grows his units on a cornfield right in the storm of battle. Whatever works for you, man. So you are building your army (yup, you guessed it, it is a deckbuilding game!), choose your enemy and try to bring the pain to him, before he brings the pain to you.
Between you and your enemy there are 2 tracks: one that your armies use to travel towards enemy, and a similar one where your enemy’s units travel to you. Again, just imagine whatever you want – two epic castles in a war or a slow, magic-fest between old, smelly wizards with rheumatism.
When you play a card, you can play it as an instant and get a trigger effect. Alternatively, you can play it as a creature on your track, then its passive ability will work.
So, you can build powerful combos, you can zombie-rush, you can build your economy and all sorts of other cool stuff. But whenever your enemy hits you, you will lose cards from your resources, so there is that. No cards to draw? You lose.