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 flush, the highest hand for the Hold’em of Omaha and many other variations. A straight of the same color means that for the straight to be called a "straight flush", the straight must be all spades, all hearts, all diamonds or all clubs. The straight flush will beat the square (quads) and to beat it you will need a stronger straight flush. So if flop is 5-6-7 all heart and you have 3-4 heart, you will only lose to 8-9 heart.
Straight flush, a rare hand!
To make a Straight flush, it's always best to have matching cards, as this will increase your chances. Imagine that you have 4-5 in clubs. How often will you hit a straight flush on flop? 0.02% of the time on flop, a little more if you look at the whole board! If you count the 5 cards (flop + turn + river), it's slightly more, but it's not every day you'll make a straight flush!