This game would be much better if it wasn't so terribly unbalanced.
There are tanks of different tiers. You start out with a pitiful little lump of metal at the lowest tier and end up with massive killing machines at the later tiers. All upgrades and new tanks are bought with experience/credits you gain from the battles you play.
Where the game really screws up though is the balance. As you get a little higher up in the tiers (Im currently at tier 4) you get thrown up against T6 tanks for some reason that I just cannot understand. In reality this means that you have literally no chance. No really, every shell you fire does 0% damage and they can patiently take aim and one-shot you.
Now if you're lucky enough to get into a group with more or less all tanks of the same tier then there are other frustrations as well. This game is free2play but it's also pay2win, there are tanks, special ammo and other perks that you can only buy for real money and while these tanks are on the same tier as you they are significantly better than you. Think something along the lines like you having to shoot them 8 times while they only have to shoot you 2 times.
In my opinion it's a real shame. If all the gear in the match rounds was evenly matched it would be so much more fun and so much better in general.
So in short, if you frustrate easily or if you don't want to have to pay in order to win, give this one a miss...