First lets clarify something, acid washing concrete or cement is quite different than How to acid stain . Although these terms are often confused they both have two different uses and outcomes.
"Acid" when refering to concrete usually means muriatic acid or hydrocloric acid. This type of acid is used to etch or 'burn' into cement based surfaces. It can be used on its own or as part of a coloration system called acid stain. Click to learn more of the decorative use of acid based stains.
Acid washing is the means of using acid to etch and prepare a cement surface for painting or the application of some type of coating. Acid is usually mixed with water and then applied to the surface of the concrete and scrubbed in. This will etch and roughen the surface so that a coating or topping will adhere to its surface. Generally this will leave no decorative color or finish behind. Acid washed or acid etched surfaces can leave some level of white staining or residue which can be very difficult to remove. It is important to work in small areas and not allow the etched residue to dry.