Acetaminophen (more properly, Paracetamol, but Acetaminophen is almost always used in the U.S. to describe the drug) is a painkiller and antipyretic usually sold under the trade name Tylenol. It's often combined with opiates, such as codeine (Tylenol-3). It does not cause the same gastric symptoms as aspirin, and is the preferred analgesic for persons suffering from gastrointestinal problems. However, it does not have the same anti-inflammatory properties as aspirin or ibuprofen, and is contraindicated for inflammatory pain conditions such as arthritis. In addition, it is somewhat more toxic than other analgesics, particularly to the liver. As such, it's not recommended for daily use. It also is not recommended for heavy usage when a patient drinks alcohol as this increases the risk of liver damage.
Although generally safe when taken as directed, Acetaminophen is the number one cause of acute liver damage in the western world (beating out even alcohol) and even normal doses can result in severe liver damage. It's also the number one cause of drug overdoses in many countries. However, it does not carry the same risk of side effects as most other painkillers. It is non-addictive, it does not cause gastritis, and it does not act as a blood thinner.
Although acetaminophen was developed in the late 19th century, early clinical trials indicated, incorrectly, that its use would result in an increase in the oxidization of hemoglobin, making it less useful in carrying oxygen to the body's tissues. These clinical trials were not challenged until the mid 20th century, when it was shown the results were incorrect. It was first marketed in the 1950s as a safer alternative to aspirin, particularly for children and those suffering from gastric ulcers. By the 1980s, it was outselling aspirin.