A biosimilar are close copies of existing biologic drugs, and therefore understanding what a biologic drug is, is necessary to understanding what a biosimilar is.
Most drugs are split into two forms, small molecules and large molecules. Small molecule drugs are synthetically made chemical compounds such as Paracetamol, which consists of only 20 atoms, they often come in pill form. Large molecules (biologics) are made by living organisms and are much larger and complicated, the cancer drug Pembrolizumab contains over 20,000 atoms. At an industrial scale they are produced by cells grown in large tanks.
All small molecule drugs of the same type are identical, but all biological drugs have small variations caused by natural processing.
Biosimilars are copies of existing biologics being used in therapeutic treatment. Biosimilars use the same amino acid code as the originator drug and follow the same or very similar process in their production. Due to natural processing they are not an identical copy of the original biologic but to be approved biosimilars must show that they have the same pharmacological profile and are equally effective and safe.