He's usually depicted as having the head of a jackel, and the body of a man, but is sometimes also shown as a jackel or dog crouching. He is often thought of and depicted as evil, when really he's not, he's one of the few Gods that hasn't done something of evil, at least not that I know of.
He has many functions, he supervises the mummification of the bodies, which he first performed on Osiris' body; he recieves the mummy into the tomb and performs the Opening of the Mouth ceremony. In the Hall of Maat he supervises the weighing of the decieseds heart against the feather of Maat. He also conducts the soul to the Fields of Peace.
He's the son of Nephthys and Osiris, though some say he's the son of Set.
One legend states that once Nephthys gave birth to Anpu she abandond
him in the wilderness, and Isis found him and adopted him.
It is thought that in the early Dynasties he was worshiped as God of
the Dead, until Osiris became more popular in the 5th Dynasty, when he
took over Anpus role.
He was worshiped all over Egypt, but was the local Diety at Cynopolis,
Lycopolis, Thinis and Abydos.
The Romans and Greeks carried on worshiping him for many years, proving
to be a strong God with much influence over people.