These were the most commonly used preservatives for the longest time because of their ability to be non-irritating, very effective, and cheap. 

Until they were found in 20 human breast tumors in 2004, which went viral and created the label on your shampoo that says “No Parabens!” This has largely been debunked and you can look into the publications, which are heavily flawed.

Some flaws include the fact that the study has a small sample size (20 tumors), fails to look at control samples of normal breast tissue, and cannot determine the source of the parabens and possible contamination of samples used in the research. Thank you Washington post) As of now, the FDA has deemed it to be safe and not a topic of concern.

Despite this, the reputation of parabens have been tarnished, so cosmetic companies have to alternatives such as phenoxyethanol. If you see parabens in your cosmetics, don't panic! If it is a reputable brand, they probably know of the controversy and have picked a safe and thoroughly tested paraben to use as a preservative and to help keep the price of the product cheap!

There are different prefixes for different parabens, and these prefixes mean the specific molecule attached to a "base" paraben (and have slightly different functions).