From the turn of the twentieth century until about the end of the second world war, Sigmund Freud was considered the end-all, be-all of modern psychology. Then, and almost ever since, it's been very vogue to be critical of Freud and his theories. I'll give a lot of my psychology professors credit, when I was studying in the early 1980's, for giving Freud his due. He may have had some strange theories, but he also had some that were worthy of consideration. My favorite of Freud's theories is that of the id, ego and super-ego. It's very simple, but seems to capture the essence of human growth, development and behavior in a profound way.
Essentially, Feud said we have these three basic drives that wax and wane throughout our lives. Depending on our circumstances and our reactions to them, a person can be very id-, ego-, or super-ego-oriented. Let's start with the id, as it is the easiest to understand.
The id is the most basic of instincts. A person who is very id-oriented is very self-concerned, even selfish. The most obvious example is an infant child. A baby has needs, and little ability to communicate those needs. A baby is hungry or soiled or is, in some way, uncomfortable. Having few communication skills, the baby cries. For those who aren't parents, the sound of a baby's crying is quite annoying. This infant is very id-oriented. (S)he wants his/her needs met and that is the only concern. For a very small child, being id-oriented is not only expected, it is a sign of good self-preservation skills. An adult who is id-oriented, on the other hand, is basically just viewed as selfish.
Now, the super-ego, as a stark contrast, is very concerned with all those around him. A person who is super-ego driven is more concerned with mankind and its problems than his own issues. A person who is super-ego driven would be a prime candidate for charrity work or other giving fields of endeavor. A person who is too super-ego oriented may forsake his own needs in order to meet the needs of others. Obviously, this can be detrimental to one's health, if one doesn't also get his most basic needs met (i.e., food and shelter), as a result of too much super-ego.
Now, the ego is the balance. According to Freud's theory, a person who is very ego-driven is actually the most well-adjusted an adult can be. (As opposed to how "ego" is thought of, in the vernacular, where a person with a big "ego" is considered selfish or vain or conceited). The person who is ego driven will make sure his own needs are met, but not at the expense of others. At the same time, he will recognize that others have needs too, and will try to be of assistance, but won't sacrifice his own comfort (at least, not to the point of self-injury) to help others. The ego-driven personality is a sort of "centered" person. According to Freud, we all strive to be as close to ego-driven as possible.
If you think about it, Freud's theory of id, ego and super-ego is really a very simple concept.; But, in its simplicity, lies a lot of truth.