Alright, "agma.io" is of course very much like "agar.io" except it's is something extra. It even has things called "powerups" which is able to give you an small advantage for a amount of time. One of them are "instant recombine" and it makes you recombine right away so you don't have to wait like 1 minute to recombine (Except in the Instant servers where you recombine almost directly). Then there is these so called abilities, there are four in total and all of them hold for 24 hours, one of them are called "Freeze" which in game costs 29,000 coins and gives you the ability to freeze all of your cells right when you press "F", this can have some amazing advantages, especially if you're playing in the instant server.
But that isn't all of it, there is skins and even custom skins, and almost everything in the game is all for free(only cost in game coins), the only thing that actually cost real money is the "Gold Membership" which allows you to get extra skins, some abilities are free for as long as you have the "Gold Membership" and also extra things called wearables which is hats, swords and more that you can use equip on your skin, everything costs coins of course but there is this special wearable called "Yellow Partyhat" which is for Gold Members only.
That is pretty much about it for everything that you need to know, you'll learn later on when you play the game :)