This I found in Wikipedia:
Manusmṛti or Manusmriti (Sanskrit: मनुस्मृति), also known as Mānava-Dharmaśāstra (Sanskrit:मानवधर्मशास्त्र), is the most important and earliest metrical work of the Dharmaśāstra textual tradition of Hinduism. Generally known in English as the Laws of Manu, it was first translated into English in 1794 by Sir William Jones, an English Orientalist and judge of the British Supreme Court of Judicature in Calcutta. The text presents itself as a discourse given by the sage called Manu to a group of seers, or rishis, who beseech him to tell them the "law of all the social classes" (1.2). Manu became the standard point of reference for all future Dharmaśāstras that followed it.
According to Hindu tradition, the Manusmriti records the words of Brahma. By attributing the words to supernatural forces, the text takes on an authoritative tone as a statement on Dharma, in opposition to previous texts in the field, which were more scholarly.
The text shows the obvious influence of previous Dharmasutras and Arthasastric work. In particular, the Manu Smriti was the first to adopt the term vyavaharapadas. These eighteen Titles of Law or Grounds for Litigation make up more than one fifth of the work and deal primarily with matters of the king, state, and judicial procedure. Though most scholars had previously considered the text a composite put together over a long period of time, Olivelle has recently argued that the complex and consistent structure of the text suggests a single author. However, no details of this eponymous author's life are known, though it is likely that he belonged to a conservative Brahmin caste somewhere in Northern India.
A range of historical opinion generally dates composition of the text any time between 200 BCE and 200 CE.After the breakdown of the Maurya and Shunga empires, there was a period of uncertainty that led to renewed interest in traditional social norms. In Thapar's view, "The severity of the Dharma-shastras was doubtless a commentary arising from the insecurity of the orthodox in an age of flux."
The dharma class of texts were also noteworthy because they did not depend on the authority of particular Vedic schools, becoming the starting point of an independent tradition that emphasized dharma itself and not its Vedic origins.
The original treatise consisted of one thousand chapters of law, polity, and pleasure given by Brahmā. His son, Manu, learns these lessons and proceeds to teach his own students, including Bhrigu. Bhrigu then relays this information in the Manu Smriti, to an audience of his own pupils.
This original narrative was subdivided later into twelve chapters. There is debate over the effects of this division on the underlying, holistic manner in which the original treatise was written. The book is written in simple verse as opposed to the metrical verse of the preceding dharmasutras. Manu also introduced a unique “transitional verse” which segued the end of one subject and the beginning of the next.
The treatise is written with a frame story, in which a dialogue takes place between Manu’s disciple, Bhrigu, and an audience of his own students. The story begins with Manu himself detailing the creation of the world and the society within it, structured around four social classes. Bhrigu takes over for the remainder of the work, teaching the details of the rest of Manu’s teachings. The audience reappears twice more, asking first to ask about how Brahmins can be subjected to death, and second to ask the effects of action.