Qrystals are a universal ingame currency which can be spent on buying any unit, item or building in the game. They can also be used for speeding up or completing any ingame processes instantly.
Qrystals can be purchased with cash or received for free in the daily bonus for the 6th day of play. You can also receive free qrystals for the completion of some quests; one qrystal is also given as a reward for each new level you achieve. Another way to get free qrystals would be participating in the contests and getting them as gifts in the community.