Four jars used in the funerary rituals to preserve the viscera of the deceased after embalming. These vessels were normally made of wood, pottery, faience, cartonnage or (precious) stone.
 

Set of canopic jars dated to the 22nd Dynasty. Each jar has a distinct stopper, shaped as the head of the the god that protects the contents of the jar.

Each jar was dedicated to a specific deity, often refered to as a son of Horus, and contained the embalmed remains of specific organs.

Thus the jar containing the liver was under the protection of Imsety, the jar containing the lungs had Hapi as a patron, the jar for the deceased's stomach was protected by Duamutef and the intestines by Qebehsenuf. 

The names of the protective deities were often written on the jars, along with a magic formula to invoce the powers of the gods. Sometimes, the stoppers of the jars would be shaped as their heads: Imsety's head was that of a human, Hapi's that of a baboon, Duamutef's that of a dog and Qebehsenuf's that of a falcon. In some cases, the stoppers of the jars were all shaped as human heads.

The jars were normally placed together but kept separate from the mummified body. In the pyramids built during the Old Kingdom, the canopic jars were often placed in a shallow pit near the sarcophagus. This pit was then covered with a slab.

In the tomb of Tutankhamun, the canopic jars were found in an elaborate shrine guarded by a statue of Anubis, in the so-called "treasury", next to the burial chamber. The stoppers on the jars were shaped to represent the head of the king himself, wearing the nemes headcloth.

In some more elaborate burials, as for example Tutankhamun's, the mummified organs were placed in a small mummy-shaped coffin, before being placed in the canopic jar. This way, the small coffin, often inscribed with magic formulae, provided an extra layer of protection for the intestines of the deceased.

One of the canopic jars of Tjuiu, mother-in-law of Amenhotep III, revealing the mummy shaped coffin which contained some of Tjuiu's organs.

Anubis

Cartonnage

Ushebti