1st degree burns are the least harmful because they don't damage the skin as bad. It normally happens when a child puts his hand on a stove for about 1-2 seconds. This can be easily treated with cold water or ice.
2nd degree burns are more serious as they can damage the skin badly but not as bad as 3rd degree burns. They are normally caused if a pot of boiling water would get all over you. The best way the treat this would be to go to a doctor
3rd degree burns are the worst burns and thankfully the least common. These are mostly caused by being burned severally such as being exposed to a house fire. The best way to treat this would be with lukewarm water with salt.