Power steering fluid is a thick brownish or reddish liquid that is used in the hydraulic elements of the power steering pump in a vehicle. Power steering fluid is necessary for any vehicle, and should be replenished whenever the level gets low. If you're out of power steering fluid and need to drive, there are a few substitutes that will work without damaging your vehicle.
Automatic Transmission Fluid
Some vehicles, particularly those manufactured between the years 1980 and 2000, can use automatic transmission fluid (ATF) as a substitute for power steering fluid. If the power steering fluid in your vehicle has a reddish or purplish colour, it is very likely ATF. You should consult your owner's manual or the car manufacturer if you are unsure.
DEXRON Transmission Fluid
DEXRON is a different type of transmission fluid, and usually has a greenish, brownish or greyish colour instead of the bright red or purple that is typical of ATF. If the power steering in your vehicle doesn't have any red or purple colourings, it may use DEXRON-type transmission fluid.
MERCON Transmission Fluid
MERCON transmission fluid is very similar to DEXRON transmission fluid, and can be used as a temporary substitute if DEXRON fluid is not available.