A Straight Flush is a five-card hand containing five sequential cards, all of the same suit. Certain named hands--such as the Royal Flush or the Wheel--are actually Straight Flushes that have earned nicknames.
It can be tempting to bet it all on a Straight Flush draw, because, hey, even if you don't hit the big hand, you've got other outs: a Straight or a Flush. But be careful--if you have a Straight Flush draw with low cards (such as the 3, 5, 6, and 7 of Hearts), you might get beat by someone else with a stronger flush--you might even get beat by someone with a stronger straight. And of course, the draw just might not hit. Never over-invest in a draw, even the very best kind.
Most of the time, the Straight Flush will be the nut hand: you've probably got everyone else beat. This means you need to build the pot by betting in a way that gets calls. Maybe you play slowly, or maybe you bet big to look like you're bluffing.
But of course, Straight Flushes aren't invincible: just like with Straights, you might end up with the ass-end, and lose to someone with the higher Straight Flush.
Straight Flushes are ranked like Straights: whoever has the higher straight wins. So, a Straight Flush to the 8 (4, 5, 6, 7, and 8 of Spades) beats a Straight Flush to the 6 (2, 3, 4, 5, and 6 of Spades).
Two players with a Straight Flush to the same high card will split the pot. There's no tie-breaker between suits.