Insulin is something that the body produces naturally that helps to control blood sugar levels and keep said levels at a normal rate. When a human cannot produce insulin and have higher blood sugar levels than normal or than is healthy, most likely diabetes is the cause, and then that person needs to be treated for it. Insulin is the main treatment for diabetes, no matter what type.
Insulin is usually treated by injection into the body, most of the time on a daily basis. A lot of the time diabetics have to inject themselves more than once a day to keep the level of insulin where it needs to be. Often, diabetics also have to eat extra during the day and have other types of monitors that help them to measure where their blood sugar levels are in case of an emergency, where in such cases insulin will be needed immediately or in worse case scenarios they might end up in the emergency room.
Insulin is usually found as a solution to be used as an injection into the body. While the injection of insulin helps to control blood sugar, it does not in fact cure diabetes. That being said, it is the main medicine for it that is prescribed by doctors and the main medicine that helps diabetics to feel better and to prevent them from getting worse illnesses as they grow older. It is also not a medicine just for when they feel sick, but for all the time, even when they are feeling well.
Many types of insulin exist, but human insulin is the most common one, and the one that appears as clear as water. If the insulin is not clear or is cloudy, a person should not use it and should consult a doctor immediately.
Altogether, insulin is an injectable solution (i.e. Novolog) that helps people with diabetes control the blood sugar levels in the body. Many different types of insulin exist and can be used depending on the person, but most of the time human insulin is what is used as an injection multiple times a day.