What were some key points about "duty" in the writings of Cicero?

Cicero was an ancient Roman philosopher who wrote many influential texts. He is today regarded as a master of eloquence. Cicero's writing interests spanned a variety of subjects, including law, nature, humanism, astronomy, and governance. The theme of "duty" comes up in much of his works, including De Officiis (On Duties in English), a work on ethics, which he broke into three separate books. The three books focus on the ideas of honor, personal advantage, and what to do when the two conflict with one another.
Generally speaking, throughout his writing Cicero takes the idea of "duty" or "obligation" very seriously. A key and recurring theme of his writing is the idea that to fulfill one's duties is to lead an honorable and virtuous life. Many of the duties about which Cicero speaks ideally lead to the improvement of one's life. For instance, in fulfilling one's duty, one may expect to attain wealth, grow personal and professional power, or achieve political advancement. In these senses, Cicero notes that men are often rewarded for fulfilling their duties.
Cicero also spends a good amount of his writing piecing out what defines honorable duty, in contrast to impure obligation. Cicero argues that there is a difference between what is honorable and what is useful useful, stating that the former is for the public good, while the latter is for the private good. He then breaks down the differences even further, comparing and contrasting "honorable" and "disgraceful," as well as "useful" and "useless."
Throughout his studies of personal "duty," Cicero continually attempts to define ideal public behavior, breaking down human personas into four main roles: the universal, the individual, the accidental or situational, and the self-chosen. Cicero argues that these four pieces of each person balance each other in varying and particular ways.
http://people.wku.edu/jan.garrett/302/4persona.htm

http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A2007.01.0048