The liver is an organ in the abdomen. It is part of the digestive system. Sometimes people use hepar- or hepat- as a prefix when they talk about the liver.

Functions 
The liver is the body's chemical factory. It does many important things:
 The liver produces (makes) bile. This is a bright yellow-green liquid that goes into the small intestines to help digest the big chunks of food we eat.
 The liver stores glucose when we eat and then puts the glucose into the blood when our blood glucose level goes down. 
 The liver takes protein and fat and turns it into glucose. This is important if we have no food to eat. We can use the fat we have saved, and make it into glucose to use.
 The liver also makes some fats and cholesterol
 The liver metabolizes (breaks down) many things in the blood:
 hemoglobin
 proteins like enzymes, insulin, and serum amyloid A
 ammonia
 toxins (substances that are poisons) and waste from the body
 The liver stores (keeps) vitamins and minerals.
 The liver makes many proteins:
 proteins that make your blood clot – called coagulation proteins
 proteins like albumin
 In fetuses when they are very small, the liver makes red blood cells
 In the liver the gallbladder stores the bile juice and then sends it out to the food that will later go in to the small intestine.

Liver diseases 
There are many different liver diseases.<ref> Dancygier, Henryk 2010. Clinical hepatology, principles and practice of. Springer. pp. 895–. ISBN 978-3-642-04509-7</ref> Liver disease can make someone very sick because of all the important work the liver does. People who have bad liver disease usually die unless they can get a liver transplant. This is when the liver from someone who has just died is put in another person by surgery. Such surgeries are usually technically challenging but can be life-saving.

 Symptoms 
The symptoms of liver disease happen because the liver does not do the work it should. 

The liver cannot metabolize toxins and waste so these bad things stay in the blood longer. One thing that builds up is a substance called bilirubin. When red blood cells die, the hemoglobin in them leaks into the blood. The hemoglobin becomes bilirubin (a yellow substance that makes bile yellow). The liver takes the bilirubin out of the blood and puts it into the bile. The bile goes into the intestines and then goes out with the waste from your body. If the liver is hurt, it does not remove the bilirubin, so the bilirubin stays in the body. This makes the person's body look yellow and is known as jaundice. So yellow eyes and yellow skin are symptoms of liver disease.

Other symptoms of liver disease are:
 Bleeding because the liver does not make enough coagulation proteins (to make clots)
 Swelling of body. If the swelling is of the abdomen it is called ascites Confusion and acting very tired because of the extra ammonia that the liver cannot metabolize
 Bleeding from big swollen veins in the esophagus called esophageal varices. If these bleed, it can cause the person to die very quickly.

 Types 
Hepatitis is when liver cells get inflamed (swollen, or increased in size). This can be from virus infections. This can be caused by toxins or poisons. The most common toxin to cause hepatitis is alcohol. It can also have genetic, or autoimmune causes, when the body's immune system hurts itself.

Cirrhosis is caused by death of liver cells that happens again and again. When the cells die, scar tissue forms. This scar tissue damages the structure of the liver. This makes the liver not work as well. But it also makes the pressure in the veins that go to the liver very high. This high pressure makes esophageal varices. The most common reason for cirrhosis in the world is hepatitis B virus infection.

Some diseases cause bad things to build up in the liver. Hemochromatosis causes extra iron to build up in the liver. Wilson's disease causes extra copper to build up in the liver. Both of these diseases hurt the cells and can cause very bad liver disease that kills people.

You can also get cancer of your liver. This can be metastatic cancer that came from some other place in your body. The liver is a common place to get metastases because it takes bad things out of the blood. So it takes cancer cells out of the blood and they grow in the liver. Cancer can also grow in the liver. If it grows in the liver it is called hepatocellular carcinoma'' (liver cell cancer). Most hepatocellular carcinoma is from cirrhosis.

Treatments 
Some liver diseases can be treated easily with medicine. Hemochromatosis is treated by taking (removing) blood from patients at intervals (times) based on the seriousness of the disease in amounts about equal to what would be taken from a normal blood donor (~470 ml).

Some liver viruses can be stopped before they start. Two types of viral hepatitis can be stopped with an immunization. Hepatitis A and Hepatitis B can be prevented with a total of five shots in a person's life. 

Some liver diseases can only be treated by a liver transplant.

As food 

The liver from livestock is a source of protein like muscle meats. It also a great source of some micro-nutrients. The flavor might be strong for some people, so it might be slightly over-seasoned. It might be toxic if you eat way too much of it.

References

Other websites 
Liver enzymes
Liver
Liver Disease

Anatomy of the digestive system
Metabolism
Organs