I like this definition of ego states:

"Paul Federn (1952), a close associate of Freud’s,... believed that the personality was not simply a collections of perceptions, cognitions, and affects, but that these organized into clusters or patterns, which he called ego states. An ego state may be defined as an organized system of behavior and experience whose elements are bound together by some common principle. When one of these states is invested with ego energy, it becomes “the self” in the here and now. We say it is “executive,” and it experiences the other states (if it is aware of them at all) as “he,” “she,” or “it,” because they are then currently invested with object energy.

Ego states may be large and include all the various behaviors and experiences activated in one’s occupation. They may be small, like the behaviors and feelings elicited in school at the age of 6. They may represent current modes of behavior and experiences or, as with hypnotic regression, include many memories, postures, feelings, etc. that were apparently learned at an earlier age. They may be organized into different dimensions. For example, an ego state may be built around the age of 10. Another one may represent patterns of behavior toward father and authority figures and thus overlap on experiences with father at the age of 10. Behaviors to accomplish a similar goal may be uniquely different from one ego state to another, especially in true multiple personalities.”

I think everyone has different ego states (and I think it is considered "normal", whatever that means, and is not a medical diagnosis).

A few months ago I recovered some repressed memories and with them, a little girl ego state of about 4 years old, who had experienced some bad stuff I had completely blocked from my mind for decades. My therapist taught me to take care of her. I have a lot of other ego states too, of different ages and "themes." Some of them are lots more distinct than others. Many are fairly integrated into my main self, others not so much. Sometimes they appear in my dreams, and sometimes in my conscious thought, and sometimes I can see them out of the corner of my eye, like a little girl shadowing me. But if I turn and face her directly, she is gone. Here is what I wrote in my journal about my ego states a few months ago:

I had a vision of who I am.... I saw myself as a room full of shadowy people, all myself, of different ages and characteristics. They are mostly gray, shadow people—all my ego states. A few are more differentiated than others and stand out to me. I recognize them. In addition to my recently found little 4 year old girl, who is with me almost constantly now, there is an older girl of perhaps 7 or 8 or 9. And there is a teenage girl too. I realized I had a new conception of self—my room full of ego states. And I also had a new conception of who I am—the team leader who holds us all together, who leads us all through life in a coordinated effort to function. And I felt proud of myself, that I could marshal the forces and get us through life. I think it’s not easy, especially under trying circumstances like the last few years. But I’ve done it.

I feel more whole now with my little girl ego state known to me. And I feel richer, like my self is richer and fuller. And I feel somehow more content than before. Like, how could I not have known this girl before because she is a part of me? And now she’s come home to me and I welcome her into my arms.

I don't have any experience with DID, but I think it is pretty different. My ego states never take me over, they are experienced more as "she" than as "I."