The Ba was always shown as a bird with a humans head, and could be seen hovering over the deads mummy or leaving and entering tombs at its own will.
The word ba is directly translated as "soul" or "spirit", but really, ba is probably better translated as "spiritual embodiment." The Ba was actually thought to have been part of the make up of a persons body and appears at death. The Ba will then take on the dead persons spirit and personality. There is a complex belief system to how a human body and spirit was constructed, and the Ba played an important part in its make up.