The ego — from the Latin for "I" — is the constructed identity: the sense of being a particular, continuous, bounded self that persists through time. It is assembled from the accumulated experiences of a lifetime, from the responses of others (particularly early caregivers) to one's existence, from the cultural frameworks that provide meaning, from the roles one plays and the stories one tells about oneself. It is not given — it is built. And what is built can be rebuilt, modified and eventually seen through.
The personality — from the Latin persona, the mask worn by actors in ancient theatre — is the outward expression of the ego: the characteristic patterns of thought, feeling and behaviour through which the constructed self meets the world. Where the ego is the interior structure of self-concept, the personality is its exterior presentation. The persona — Jung's term for the social mask — is the most outward layer of all: the face presented to different audiences in different contexts, which may bear only a distant relationship to what lies beneath.
The ego is not a thing but a process — a continuous activity of self-construction and self-maintenance. It is the activity of "being someone" — of maintaining the story of a coherent, continuous self across time. This activity consumes enormous energy: keeping the story consistent, defending it against contradictory evidence, managing the gap between the self-image and the actual experience. Much of what is called psychological suffering is the ego's effort to maintain a self-concept that no longer fits the reality of the person's experience.