Erik Erikson, a Freudian ego-psychologist who is also socially-oriented, is best-known for his ego development. It's an elaboration of Freud's five stages of development, which didn't include adulthood. Erik Erikson's ego development has eight stages that extend from infancy to late adulthood.
The first stage of Erik Erikson's ego development is Trust vs. Mistrust, taking place in infancy. If the infant's needs are dependably met, they will develop that crucial basic sense of trust. Therefore, the parents must make those efforts to touch their infant and be in their line of vision, because without this sense of security, the infant will learn to not trust their new world.
The second stage is Autonomy vs. Shame and Doubt, occurring in toddlerhood (ages 1-2). Toddlers are learning to exercise free will and be independent. Not only are they learning to feed themselves and communicate verbally, but they are also learning to be potty trained. If toddlers aren't acknowledged or rewarded with a big smile and a "good job!" for learning new skills, they'll develop a lower self-esteem. For example, if a toddler who is learning to be potty trained accidently goes in their underwear, but then runs to the toilet after realizing what he or she has done, they should be encouraged rather than yelled at since they are grasping the concept of potty training (even if they can't completely control their bladder yet).
The third stage is Initiative vs. Guilt. Preschoolers, from ages 3-5, are now learning to initiate tasks and carry out plans. Pretend play and asking questions are the fundamentals of a preschooler who has a high self-esteem. If a preschooler's questions or made up stories are constantly shut down by their parents, they may start feeling guilty for even coming up with them, and start developing a lower self-esteem. It's important to encourage a preschooler's intuitive mind so they don't feel guilty about learning new things (or interrupting mommy now and then to resolve whatever's on their mind).
The fourth stage is Industry and Competence vs. Inferiority. This stage covers elementary school years, from kids 6 years old to those entering puberty. Children should be learning the pleasure of applying themselves to tasks, and be learning the pleasure of knowledge. Those who start feeling inadequate to the teacher or their peers instead develop a sense of inferiority. It's important to target those children who may have difficulties with certain subjects as soon as possible to prevent that sense of inferiority. A struggle with learning multiplication tables could damage a child's confidence when it comes to learning division, thus hindering their performance and speed of learning.
The fifth stage, and probably the most important, is Identity vs. Role Confusion. This is the infamous battle adolescences face as they try to figure out who they are and what they want to be. This could explain why teens are known to make the most mistakes; they are constantly testing out new roles, learning which ones suit them and can be integrated into the person they want to become. Those who are given the freedom to test out these roles often come to their conclusions sooner than those who are oppressed by parents telling them, "this is what you're going to do." It's crucial for teens to figure out themselves on their own so they can feel that sense of self-actualization rather than confusion and conflict. This is also the best stage for adolescences and even parents to try and fix the mess-ups from the previous stages. The sooner the adolescence re-discovers confidence, the sooner they can start figuring out who they are on their own.
The sixth stage is Intimacy vs. Isolation. Starting in the 20's and ending in the early 40's, these young adults are struggling to form close relationships and to feel that intimate love, or start their families. Those who aren't successful start feeling isolated, not only from the opposite sex but also from their same-sex friends. This is why Identity vs. Role Confusion is so important - it's nearly impossible to have a successful relationship with another person if you don't know who you truly are. If you are still struggling with intimacy and you are in this stage, take some time to yourself so you can decide who you are. Don't mask your uncertainty with multiple lovers or by jumping into a relationship because it will eventually take you right back into isolation.
The seventh stage is Generativity vs. Stagnation. This is where the infamous "mid-life crisis" occurs in middle adulthood (40's through 60's). As we reach that "hump" in life, we tend to look back on what we've accomplished. If we discover a sense of contributing to the world through our families or work, we feel generativity and a sense of purpose. If we discover we haven't done all we could have, we feel a lack of purpose and become self-absorbed. In this stage, it's important to not give up even if you feel you're running out of steam - keep reaching towards your goals and find that true sense of purpose.
The final stage is Integrity vs. Despair, occurring in late adulthood. This is simply when the older adult reflects back on their life and asks, "Did I do everything I wanted to?" "Did I contribute to my family and work?" "Did I accomplish every goal I set for myself?" If they feel satisfaction in these answers, they gain a sense of integrity. If they feel as if they've failed, they instead reach despair. Those who feel they've failed may reach out one more time in effort to right their wrongs, and fear death. Those who have reached that sense of accomplishment welcome death as the completion to their lives.
References: AP Psychology notes, and Learning Place Online