Sigmund Freud was a 19th-century Austrian doctor who pioneered the field of psychiatry. His research was revolutionary for its time and generated a wide variety of theories and practices. A major subset of his theories were centered around personality: how your personality is created and influenced by biology and the forces around you.
It is important to note that Freud's theories do not have a strong backing in empirical evidence. They do, however, provide an interesting framework for observation.
The cornerstone of a Freudian personality is the id. The id is the most basic part of a person's personality -- it is solely concerned with satisfying basic, day-to-day needs. It is the most "animalistic" part of the human personality; all it cares about is getting fed, watered, procreating and spreading its DNA. For example, if you are hungry and standing at a buffet and you let your id control your personality, you would grab some food and eat it, probably avoiding cutlery and a plate and certainly not waiting until you pay. These basic desires all come from your id, and the other parts of your personality are what keep them at bay. This part of your personality is pure biology -- it is your basic, innate set of instincts.
The ego is the next layer of the personality. Your ego is what actually carries out the desires handed to it by the id; if you are hungry, it is up to your ego to recognize the feeling and go about appeasing it by finding food. So, if you were pure id and you were hungry you would not go to a buffet on your own. Rather, you would need to be placed there, at which point you would eat until the hunger is gone. The ego is what solves the id's problems by assessing what they are and experimenting until they go away. So, it is the ego that makes people able to go to the buffet on their own when hungry rather than wait to be placed there. This is also biology, as it does not develop based on social stimuli. Rather, it's a mechanism to feed the id's needs and wants.
As you grow up, you develop a set of algorithms in your brain. These are the processes that worked the most effectively to solve your id's problems. It is what teaches you that when you are hungry, you need to not only go to the buffet but also to go with money, use plates and leave some money in the tip jar. It is also what teaches you not to do certain things -- for example, take food out of the buffet tubs with your hands or cut in line.
Where the id is pure needs and wants and the ego is the capacity to answer these needs and wants, the superego is the "playbook" that makes the ego more efficient. Therefore, it is the part of your personality most influenced by the people around you. It is the "nurture" part of "nature versus nurture."
The superego doesn't develop until you are around seven years old. You develop it a little more every day by being told to do things and by seeing behaviors modeled. In Western cultures, for example, your superego develops to accommodate the fact that people use toilet paper. In some Asian and Middle Eastern cultures, by contrast, toilet paper is not used.
Freud's personality theories hinge on the superego. Your personality develops based on it, as your personality is basically the manner in which you solve problems. If you become anxious when confronted with a problem, it is because you were taught by your parents and other influences that this is the correct way to respond to a problem. If, on the other hand, you have a hard time taking problems seriously, the people around you are just as much to blame. The algorithms that you learn are what create your superego and with it your personality; everything else is just biology.