Answer by taking a quote from the following article:

Alfred Adler was born at Mariahilfer Strasse 208 in Rudolfsheim, then a village on the western fringes of Vienna, and today part of Rudolfsheim-Funfhaus, the 15th district of the city. He was second of the seven children of a Hungarian-born, Jewish grain merchant and his wife. Alfred's younger brother died in the bed next to him, when Alfred was only three years old. Alfred was an active, popular child and an average student who was also known for his competitive attitude toward his older brother, Sigmund.

Adler began his medical career as an ophthalmologist, but he soon switched to general practice, and established his office in a less affluent part of Vienna across from the Prater, a combination amusement park and circus. His clients included circus people, and it has been suggested that the unusual strengths and weaknesses of the performers led to his insights into "organ inferiorities" and "compensation".  In 1902 Adler received an invitation from Sigmund Freud to join an informal discussion group that included Rudolf Reitler and Wilhelm Stekel. The group, the "Wednesday Society" (Mittwochsgesellschaft), met regularly on Wednesday evenings at Freud's home and was the beginning of the psychoanalytic movement, expanding over time to include many more members. A long-serving member of the group, Adler became president of the Vienna Psychoanalytic Society eight years later (1910). He remained a member of the Society until 1911, when he and a group of his supporters formally disengaged from Freud's circle, the first of the great dissenters from orthodox psychoanalysis (preceding Carl Jung's split in 1914). This departure suited both Freud and Adler, since they had grown to dislike each other. During his association with Freud, Adler frequently maintained his own ideas which often diverged from Freud's. While Adler is often referred to as "a pupil of Freud's", in fact this was never true; they were colleagues, Freud referring to him in print in 1909 as "My colleague Dr Alfred Adler". In 1929 Adler showed a reporter with the New York Herald a copy of the faded postcard that Freud had sent him in 1902. He wanted to prove that he had never been a disciple of Freud's but rather that Freud had sought him out to share his ideas.  Adler founded the Society for Individual Psychology in 1912 after his break from the psychoanalytic movement. Adler's group initially included some orthodox Nietzschean adherents (who believed that Adler's ideas on power and inferiority were closer to Nietzsche than Freud's). Their enmity aside, Adler retained a lifelong admiration for Freud's ideas on dreams and credited him with creating a scientific approach to their clinical utilization (Fiebert, 1997). Nevertheless, even regarding dream interpretation, Adler had his own theoretical and clinical approach. The primary differences between Adler and Freud centered on Adler's contention that the social realm (exteriority) is as important to psychology as is the internal realm (interiority). The dynamics of power and compensation extend beyond sexuality, and gender and politics can be as important as libido. Moreover, Freud did not share Adler's socialist beliefs, the latter's wife being for example an intimate friend of many of the Russian Marxists such as Leon Trotsky.  Following Adler's break from Freud, he enjoyed considerable success and celebrity in building an independent school of psychotherapy and a unique personality theory. He traveled and lectured for a period of 25 years promoting his socially oriented approach. His intent was to build a movement that would rival, even supplant, others in psychology by arguing for the holistic integrity of psychological well-being with that of social equality. Adler's efforts were halted by World War I, during which he served as a doctor with the Austrian Army. After the conclusion of the war, his influence increased greatly. In the 1930s, he established a number of child guidance clinics. From 1921 onwards, he was a frequent lecturer in Europe and the United States, becoming a visiting professor at Columbia University in 1927. His clinical treatment methods for adults were aimed at uncovering the hidden purpose of symptoms using the therapeutic functions of insight and meaning.  Adler was concerned with the overcoming of the superiority/inferiority dynamic and was one of the first psychotherapists to discard the analytic couch in favor of two chairs. This allows the clinician and patient to sit together more or less as equals. Clinically, Adler's methods are not limited to treatment after-the-fact but extend to the realm of prevention by preempting future problems in the child. Prevention strategies include encouraging and promoting social interest, belonging, and a cultural shift within families and communities that leads to the eradication of pampering and neglect (especially corporal punishment). Adler's popularity was related to the comparative optimism and comprehensibility of his ideas. He often wrote for the lay public. Adler always retained a pragmatic approach that was task-oriented. These "Life tasks" are occupation/work, society/friendship, and love/sexuality. Their success depends on cooperation. The tasks of life are not to be considered in isolation since, as Adler famously commented, "they all throw cross-lights on one another".  In his bestselling book, Man's Search for Meaning, Dr. Viktor E. Frankl compared his own "Third Viennese School of Psychotherapy" (after Freud's and Adler's schools) to Adler's analysis:  According to logotherapy, the striving to find a meaning in one's life is the primary motivational force in man. That is why I speak of a will to meaning in contrast to the "pleasure principle" (or, as we could also term it, the will to pleasure) on which Freudian psychoanalysis is centered, as well as in contrast to the will to power stressed by Adlerian psychology.  Adler often emphasized one's birth order as having an influence on the style of life and the strengths and weaknesses in one's psychological make up. Birth order referred to the placement of siblings within the family. Adler believed that the firstborn child would be in a favorable position, enjoying the full attention of the eager new parents until the arrival of a second child. This second child would cause the first born to suffer feelings of dethronement, no longer being the center of attention. Adler (1908) believed that in a three-child family, the oldest child would be the most likely to suffer from neuroticism and substance addiction which he reasoned was a compensation for the feelings of excessive responsibility "the weight of the world on one's shoulders" (e.g. having to look after the younger ones) and the melancholic loss of that once supremely pampered position. As a result, he predicted that this child was the most likely to end up in jail or an asylum. Youngest children would tend to be overindulged, leading to poor social empathy. Consequently, the middle child, who would experience neither dethronement nor overindulgence, was most likely to develop into a successful individual yet also most likely to be a rebel and to feel squeezed-out. Adler himself was the third (some sources credit second) in a family of six children.  Adler never produced any scientific support for his interpretations on birth order roles, nor did he feel the need to. Yet the value of the hypothesis was to extend the importance of siblings in marking the psychology of the individual beyond Freud's more limited emphasis on the mother and father. Hence, Adlerians spend time therapeutically mapping the influence that siblings (or lack thereof) had on the psychology of their clients. The idiographic approach entails an excavation of the phenomenology of one's birth order position for likely influence on the subject's Style of Life. In sum, the subjective experiences of sibling positionality and inter-relations are psychodynamically important for Adlerian therapists and personality theorists, not the cookbook predictions that may or may not have been objectively true in Adler's time.  For Adler, birth order answered the question, "Why do children, who are raised in the same family, grow up with very different personalities?" While a strict geneticist, believing siblings are raised in a shared environment, may claim any differences in personality would be caused by subtle variations in the individuals' genetics, Adler showed through his birth order theory that children do not grow up in the same shared environment, but the oldest child grows up in a family where they have younger siblings, the middle child with older and younger siblings, and the youngest with older siblings. The position in the family constellation, Adler said, is the reason for these differences in personality and not genetics: a point later taken up by Eric Berne.

What were his ideas on birth order?
Adler often emphasized one's birth order as having an influence on the style of life and the strengths and weaknesses in one's psychological make up.