Dragon coins are the on site currency used to buy items from the various shops on ToD or buy other player's dragons. These are not to be confused with points, which are used exclusively in the forum shop.
Dragon coins are very valuable as they give you access to the various alt potions as well as exclusive dragons not found in the main lair.
These shops are the primary output of dragon coins.
How to Earn Dragon Coins
The easiest way to earn dragon coins is by clicking other user's dragons. You can click a dragon once per day, earning 10-20 a click. The amount you get is randomly generated. Trading dragons via the Dragon Shop can also be lucrative.
Going through the paged Coin Lair under the explore tab is the best way to make sure you click every egg and hatchling on the site without accidentally wasting your time clicking on adoptables you've already collected on. Going through about 25 pages can net you around 10,000 !
But if you're in a hurry and just want a quick burst of coins, using the non paged coin mine is faster since you can click more rapidly. However, due to the nature of this coin mine's setup, you will occasionally re-click the same adopts more frequently if you click though many pages. When none of the adoptables on the page have been clicked you average around 1,000 per page.
The final way to earn coins is by donating to the site. In order to keep the site up and running, you can donate by clicking the dragonfly dragon at the bottom of the site. Not only do you contribute to keeping the site alive, but you can also recieve coins or a special donation dragon.