Royal flush
A royal flush is ten through ace all of the same suit, the highest straight flush there is and therefore the best hand in poker.
It is not a separate hand class, it is the top end of the straight flush. There are 40 straight flushes among the 2,598,960 five card hands and 4 of them are royal, one in each suit.
Because it cannot be beaten or tied by a better hand, the only difficulty it presents is getting paid. Two royal flushes cannot exist at the same table, so the pot is yours in full whenever it appears.
an example
your cards
board
Your ace and king of hearts complete the queen, jack and ten of hearts on the board. Nothing beats it, nothing ties it, and the only question left in the hand is how much anybody is willing to put in.
This example is made up. The real one is in your own hand history: find royal flush in your own hands and see how often it came up and what it cost you.
related terms
Straight flush
A straight flush is five consecutive cards all of the same suit, the best hand class there is.
Flush
A flush is five cards of the same suit, which beats a straight and loses to a full house.
Nuts
The nuts is the best hand possible on a given board, the one no other two cards can beat.
next
Knowing the word is not the same as seeing it.
Real spots where you say which hand wins, and it keeps score.