In Dragonvale there are 4 types of treasure: dragoncash, gems, dragon treats, and berrywhite roses.
Dragoncash comes as coins earned by your dragons. They are what you use to buy the main things in the game, such as buildings, decorations, elemental dragon eggs, and dragon habitats. To collect coins from your dragons, you can wait until the dragoncash tab pops up over a habitat, or you can tap the habitat and tap the treasure chest labeled "collect". To see how many coins you have, look at the top of the screen next to the coin icon.
Gems are more valuable treasure than dragoncash but is also a money currency (kind of like a coin is to a dollar). They are used to speed things up, buy hybrid or rare dragons, and things like that. The main difference between gems and dragoncash is that you cannot earn gems easily. While your park is level 10 or under, sometimes the Great Nogard will give you gems to complete a task that you can save instead of spend. You can also get gifted gems from friends using the Dragonsai Gifting Tree or earn gems by completing certain tasks.
Dragon Treats are not used to buy anything. Instead, they are used to feed your dragons and make them grow. You can get dragon treats by planting them on a treat farm using dragoncash.
Berrywhite Roses only come in the month of February during the annual collecting of roses with Garcia. They are flowers you can earn by finishing tasks, leveling up, or doing simple things like harvesting dragon treats or collecting dragoncash from your habitats. These flowers can be turned in to the Rosefall Tree to work towards getting rewards.