Generally we think of "fillers" as bulk ingredients that serve no purpose, but the filler ingredients in laundry detergents do affect the detergent. They're used to change the consistency of the detergent, to make it more pourable, more soluble and to help it disperse evenly. While different companies each have their own formulas, and consequently use more or less of these fillers, some common fillers are used in many different brands and types of detergent. The main split is between powdered detergents and the liquid varieties. Because the fillers are there to change the consistency of the product, different fillers are needed for powders and liquids.
Sodium sulfate is a salt and is a common additive to powdered detergents as well as shampoos, bath soaps and skin care products. It keeps powders from clumping, thickens when water is added and helps granules of powdered detergent flow freely. However, it does add another chemical to the mixture already used in detergents. While the Cosmetic Ingredient Review Expert Panel has found it is safe in products intended to be washed off, the fact that it caused some skin irritation in preparations meant to be left on the skin means that its use is limited in such products.
Borax, or sodium borate, is popular for homemade laundry detergents and is also used as filler in some commercial brands. It is used as a preservative because it limits the growth of bacteria and also serves much the same function as sodium sulfate when used in powdered detergents. It is used in a wide variety of products including cosmetics, lotions and bath products.
The most common filler in liquid detergents is water, which is used to dilute the chemicals and help them dissolve so that they can be more evenly distributed. Some brands use more water than others, and many brands now also sell detergents that have less water filler and are more concentrated.
In addition to sodium sulfate, borax and water, there are smaller amounts of other fillers in many detergents, both liquid and powdered. These fillers can include chemicals, such as sodium silicate, to prevent corrosion inside your washing machine, and agents that limit the amount of foam produced by the detergent. Alcohol may also be used in liquid detergents to increase the water solubility and lower the detergent's freezing temperature.