Flex refers to how much the shaft of the golf club bends during the swing. There are generally five ratings for flex: extra stiff, stiff, regular, senior, and ladies. Your optimal flex will vary based on your technique and driver distance.
As shown here:
Flex
Driver Distance
Extra stiff
Over 275 yards
Stiff
240-275 yards
Regular
200-240 yards
Senior
180-200 yards
Ladies
Under 180 yards
Iron sets typically include seven or eight clubs consisting of 4, 5, 6, 7, 8 and 9 irons in addiiton to a pitching wedge and sometimes a gap wedge.