'Parabens' are a family of synthetic preservatives (methylparaben, propylparaben, isoparaben, and butylparaben). Parabens are very effective as a broad-band preservative and found in almost every common beauty care product including color cosmetics, shampoos, conditioners and lotions. Beyond being effective they also allow companies to easily and very cheaply preserve products, allowing them to have shelf lives of 2-3 years without spoiling.
Paraben preservatives are suspected carcinogens. Multiple studies related specifically to breast cancer have discovered parabens in breast cancer tumors. Paraben preservatives as well as other synthetics commonly used in cosmetics and deodorants are absorbed into the skin and accumulate in the body.
As the use of advanced synthetics becomes increasingly popular in almost all personal care products, our society is absorbing and accumulating ever greater amounts of synthetics.
Some anti-paraben advocates call this accumulation a 'body burden' and encourage the public to avoid paraben preservatives and other identified toxic ingredients to decrease your body's overall chemical burden over a lifetime of use. Theorists believe that the sum of all the chemicals you absorb that build up in your system may eventually be one of the triggering agents in the development of cancer.