as above. The royal flush is a case of the straight flush. It can be formed 4 ways (one for each suit), giving it a probability of 0.000154% and odds of 649,739 : 1.
The Royal Flush is top on the list of poker hand rankings. This is the strongest possible hand in poker and can never be beaten. It is made when we have the Ace-high straight while holding cards all of the same suit.