The Four Sons of Horus (from left to right)
Hapi, Amset, Qebehsenuf, Duamutef.

The Four Sons of Horus were, in Ancient Egyptian belief, the four protector's of the deceased's bodily organs. These organs were; the liver, stomach, intestines and lungs.
They were protected as they were thought to be the major organs in the human body, and so, when the deceased entered into Duat (The underworld) they could then still be able to breathe & use their other organs. One of the major organs, the brain was not protected as the doctors of that period did not realise the importance of it and did not know it's function, which explains why, in the numerous Tombs in the Valley of the Kings, the brain is not preserved in canopic jars.
Canopic Jars were the storage jars for the mummfied organs, these could be decorated as either; the Sons of Horus, or they could be of people who were significant in the deceased person's life.
The Four Sons of Horus, were, themselves not only protectors, but they were also protected by the Goddess's. The Goddess's were Isis (wife/consort to Osiris), Nephthys (sister to Isis & Osiris and wife/consort to Set), Neith & Selket. Amset was protected by Isis, Duamutef by Nephthys, Hapi by Neith & Qebhsenuf by Selket.

Sometimes though, the canopic jars didn't feature the heads of the Four Sons, instead they could sometimes feature the heads of those significant to the lives of those now dead. An example of this is the above image, featuring the top of a Canopic jar found in Tutankhamen's Tomb. It features Akhenaten's second wife (and also thought to be Tutankhamen's mother), Kiya. Kiya being Tutankhamen's mother is only speculation as not much is known of Kiya but more can be read on Kiya in Religious Revolution!