In Realm Grinder, Coins represent the basic resource. There are Gold coins, Diamond coins, Emerald coins and the special Faction coins.
Gold Coins are the basic income used until R39. They are the first resource the player gains by clicking on the background, and enable the player to buy their first building. Once a building is bought, it will generate coins and start the automated coin-gain process, allowing the player to buy more buildings and upgrades, which will finally start to generate gems.
Diamond Coins replace Gold Coins after ascending at R40. They were initially implemented during the v2 Graphic Overhaul expansion to overcome technical problems computers have with very large numbers.
Emerald Coins replace Diamond Coins after ascending at R100. They were initially implemented in the 3.0 Chaos Leges update.
Your total Coin Production per Second, and per Click, can be seen by hovering over the hourglass icon.
In the stats., you can also see your Coins per Click, Coins Gained, Coins Gained (max), and Coins Gained by Clicking, for this game or in total — "total" meaning "this reincarnation".
There are dozens of coin trophies for gaining certain threshold of coins.