You will not need one if your transit time/layover will be less than 48 hours (must show proof of onward flight ticket). Leaving the airport may be permitted but you will have to surrender our passport. You will need to inquire with your airline, as many offer short duration tours to their passengers with transit times between 6 and 48 hours.
If your transit time is less than 6 hours, you may leave the transit area, but not the airport.
The above DOES NOT apply to nationals of Iran (who always require a transit visa) and Afghanistan, Lebanon, Palestine, and the Philippines, who must remain in the transit area at all times.