Insulin is a small protein made up of multiple amino acids that are linked together. It is manufactured by beta cells in the islets of Langerhans in the pancreas. In humans insulin has a very short duration of action and is released continuously. When we eat foods that contain carbohydrates and sugars, a large spike of insulin is released.
By making small changes to the amino acids insulin manufacturers have been able to produce variations of human insulin called analogues that have specific characteristics that help with the delivery of insulin as a medicine. Modifications to insulins such as Humalog, Novolog/Novorapid, Apidra or Fiasp can for example when given before food speed up insulin delivery to help regulate blood sugars. It can also be more slowly released from under the skin so that insulin is absorbed continuously as a basal, long acting or background insulin.