The characteristic of a double action in a revolver is that it can be fired without manually cocking the weapon like the single action. Pulling the trigger from a hammer-down stance requires a long and hard pull, which makes it difficult for newbies to aim down the sight harder at the time of firing. Many users of double of action revolvers prefer to use it as a single action revolver by pulling down the cock manually but prefer the option of firing the weapon un-cocked should the need arise.
There are also semi-auto double action firearms that allow the weapon to be fired from the hammer-down state. If the firearm supports single action fire, the remaining shots after the first round are all cooked using the kinetic energy from the recoil. Semi-autos using double action that cannot be fired in double action are referred to as DAO or double action only.
Most semi-autos and small revolvers built to be easily concealable are DAO. This eliminates the chances of accidental discharge, and since there is no need for the external hammer, it cannot snag while being drawn.