In Texas Hold 'em, having 2 and 7 off suits is rated the worst hand. This is because they're the two lowest cards you can have, and they can't create a straight (there are five cards between 2 and 7). Even if they're suited, they'll give you a low flush, and if they both create pairs, it'll be a poor hand.
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.