The Egyptians believed that every person was composed of three essential elements: body, ba, and ka.
The body is the physical body and is unique to each individual. As a person gets older, so the body ages and changes - the Egyptians' expressed the idea of growing up as a process of "making changes" - and death is the last change.
Each person also has a ba. Though the ba is also unique to each individual, it is not a physical entity. Ba is sometimes translated as "manifestation," and can be thought of as the sum total of all the non-physical things that make a person different from others. In this sense, ba is very similar to what we call "personality" or "character." In the afterlife, the ba is represented as a bird, often with a human head.
Each person also has what is called a ka, or life-force, and it is the ka which is the difference between being alive and being dead. Unlike the ba, the ka is not individual, but common to all living people and the gods: in the beginning, the creator made ka, and ka enters each person's body at birth. Like the ba, the ka is not a physical entity, though it has a definite physical connection. In the plural, ka means "sustenance," linking it to the idea of food. In fact, ancient Egyptians would bring food to a dead person's tomb as an offering to his or her ka. But since the ka is not strictly physical, the food was not there to be literally eaten by the deceased or the deceased's ka, but it was the life-preserving force in the food that was being offered.
When a person dies, so the Egyptians believed, the ba and ka become seperated from the body, though they do not die. In the New Kingdom period and after, the Egyptians effected this seperation through the Opening of the Mouth ritual, in which the ba and ka are released to go to the next world.
In the next world, or underworld, the goal is to live with ones ka. In order for this to happen, the ka needs to be summoned back to the body and recognize it. But since the body is bound in its wrappings, it must rely on its ba to seek out its ka. During the nightime, when the sun god, Ra, is said to visit the underworld, the ba may roam freely in the underworld, or to popular places in this world, but it's anchor in this world, where it must return when Ra leaves the underworld, is the body, because together they are part of the same whole being.
In seeking a union with the ka, the ba must overcome many potential dangers in the underworld. But if it does succeed, it will reunite with the ka and form what is called akh. The Egyptian's believed that there are only three kinds of beings that inhabit the hereafter: the dead, the gods, and akhs. Akhs are those who have successfully made the transition to new life in the next world, where they live with the gods. The dead are those who have failed to make the transition. It is said that they have "died again," with no hope of renewed life.