Definition of straight flush
A straight flush is a sequence of 5 cards of the same suit (spades, clubs, hearts or diamonds). The lowest straight flush is A-2-3-4-5 and the highest is T-J-Q-K-A for the royal flush, the strongest hand in Omaha Hold'em and many other variations. A straight of the same suit means that for the straight to be called a "flush", the straight must be all spades, all hearts, all diamonds or all clubs. The straight flush will beat the four of a kind (quads) and to beat it, you will need a stronger straight flush. So if the flop is 5-6-7 all hearts and you have 3-4 in hearts, you will only lose to 8-9 in hearts.
Straight flush, a rare hand!
To get a straight flush, it is always better to have matching cards, it will increase your chances. Imagine you have 4-5 in clubs. How often will you hit a straight flush on the flop? 0.02% of the time on the flop, a little more if you consider the whole board! If you count all 5 cards (flop + turn + river), it's slightly more, but it's not every day you'll hit a straight flush!