The Sun in astrology is treated like a planet. The Sun in a person's natal chart represents the ego or the basic drive in life. The sign in which it was passing by at the time of birth will show more about why and how a person acts. The Sun shows the basic vitality of a persons ego as it manifests in the psyche and into the environment. It is "masculine" in the sense that it pushes out into the world. The house that the Sun is found at birth is the "where" or manner in which the Sun in a certain sign acts out. 

For example: 

Let's say that a person's Sun is Libra in the 5th house- the house that is "ruled" by Leo. The ego, therefore, will ACT in a way that is stretching itself out into world by creating relationship, harmony and beauty. Put the Libra Sun in the 5th house of Libra and the person will be inclined to act out that desire to create relationship, harmony and beauty in a creative and playful manner. This is a clear indicator of an artist. Here is also a person who is basically joyful, playful, and optimistic. Even if sometimes, it is simply for the sake of "keeping up appearances".

Another way to look at this particular example is that the soul had CREATED this chart for themselves in order to LEARN about being more playful and creative. Therefore, the chart will reflect that souls direction towards their enlightened state. It is up to ALL OF US to choose the path that is the most beneficial to our soul's calling. We are here to learn. For this example, taking in account just the Sun and it's placement, this soul would definitely be a creative force in the world. We must look to other factors such as the other planets and placements in the chart to get a better picture of the blueprint. This person may have a Saturn conjunct Mercury in Virgo for example, and may have trouble finding freedom of expression. The other side of it is that the person may find an outlet that is analytical and precise in order to fulfill the calling. Perhaps a fine artist who excels in creating very detailed portraits or a historical fiction writer who researches their subject with the most exacting measurements. These are all possibilities within this is UP TO EACH OF US to choose the correct path for us. Astrology can show us propensities for choices. 

Here is a basic breakdown of the Sun in each sign:

Sun in Aries: Here, the Sun's basic instinct is to explore be brave and carve out frontiers. One can imagine a soldier who is out in the front lines of battle and volunteers to scout out the territory. This may translate as a person who's drive in life is to bravely push the limits. Whether it be a lawyer competition for partnership or a detective who puts themselves continually on the line, the Aries Sun personality wants to push, strive and achieve. 

Sun in Taurus: The Sun's basic drive is to find serenity, stability and security. They will calmly tread the road slowly and steadily until they reach their goal all the while enjoying the material pleasures in life. One can imagine the archetype of the office worker who continually does a little better at every opportunity in order to one day achieve CEO. They are pleasure-seekers at heart and love the company of down-to-earth friends who supply plenty of laughs and practical support. 

Sun in Gemini: The Sun here has a basic drive to explore every curiosity that they come across. Depending on what house and on the other planets, this may translate as an inner drive and curiosity to study every fascinating fact, or to outwardly express their curiosity about the world as they find it. Their entire environment is fuel for their curious mind. There is an emphasis on communications in all its splendor and of collecting information. The way that they might use this information can be gleaned from the other planets and aspects in the chart. 

Sun in Cancer: Here, the Sun is cooled down by the moon ruled Cancer who's basic desire is to protect, nurture and feel deeply. There is an emphasis on protection and preservation- either of themselves or of those they love. A need to feel safe is paramount for their souls evolution. For these gentle souls, the need to feel loved and safe generates in them a natural instinct to be compassionate towards others. If the self-preservation is soley turned inward, then the person may continually "hide" behind insecurities and fagility. In their most exalted state, here is a soul who makes friends for life and protects and nurses all those around them. The United States, being a Cancer from the creation of a nation on July 4th is a perfect example of self-preservation, nurturing of it's citizens and a fixed nature on creating a haven of safety.