Dragons are the red, green, and white tiles, four of each. Chinese tables call them 三元. American tables call them dragons.
Red is 中, green is 发, white is the blank tile. American cards often print the white dragon as soap.
You can pung or kong dragons, and a line on the card may ask for them. A joker can finish identical dragons. It cannot turn a dragon into a pair.
Riichi, Hong Kong, and MCR all use the three dragons. Sichuan 血战到底 uses only the three suits.
The tiles lesson teaches dragons after the suits.
The white dragon is a dragon, not a joker and not a flower.