Acetaminophen is a pain reliever similar to aspirin, but without aspirin's anti-inflammatory abilities, or its undesirable affects on the stomach lining, and without aspirin's anticoagulatory effects.
Acetaminophen is often combined with opiates such as codeine and dihydrocodeine to form a more potent pain killer.
Unlike aspirin and other cyclooxygenase inhibitors that work on the COX-1 and COX-2 enzymes, acetaminophen works on the COX-3 enzyme, which is present in the spinal column and brain. This lets it avoid preventing prostaglandins elsewhere in the body from working. This is why it has no anti-inflammatory effects, and does not affect blood platelets or the stomach lining.