Top to bottom: hammer uncocked. Hammer cocked, safety off, ready to fire.
The hammer of a firearm was given its name for both resemblance and functional similarity to the common tool. The function of the hammer is to strike the firing pin in a firearm, which in turn detonates the impact-sensitive cartridge primer.
When the trigger of a firearm is depressed, it acts on the sear, which in turn releases the spring-tensioned hammer. Some firearms have a striker instead of a hammer and firing pin.
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The original article can be found at Hammer (firearm) and the edit history here.