Citric Acid is an organic acid existing in a variety of fruits and vegetables, but it is most concentrated in lemons, limes, grapefruit and oranges. It is an environmentally benign cleaning agent and acts as an antioxidant. Please don't mistake 'organic' for weak!
In food preparation, citric acid is used to keep fats from separating (an emulsifier), to prevent sucrose crystallization in caramel, or to impart a sour flavor for foods or candies and drinks.
Citric acid is also an excellent chelating agent, binding to most metals. It is used to remove limescale and rust. It can be used to soften water (which makes it useful in soaps and laundry detergents). Citric acid is the active ingredient in some bathroom and kitchen cleaning solutions. As stated by Verhoff, a solution with a 6% concentration of citric acid will remove hard water stains from glass without scrubbing. THIS is why we use it!
Some may find they have a sensitivity to citric acid. If so, skin contact with citric acid may cause irritation or rash. Eye contact will result in a slight burning sensation (just as the zest of a lemon would cause). In chronic exposure (large amounts over a long period of time), citric acid can lead to corrosion of tooth enamel. This can be avoided by consuming drinks containing citric acid through a straw.