The "straight flush" is the most unusual (hence most valuable) rank. You hardly ever see them during games! To have a straight flush, you must hold 5 cards that qualify both as a straight and as a flush. In other words you must hold 5 cards with consecutive values and identical suits. (Please see the sections on "Straights" and "Flushes" for more precise definitions.) Below is an example of a "Straight Flush":
The cards are all diamonds (same suit), and they have consecutive values (4, 5, 6, 7, 8). I wish I would get cards like these more often!