Manusmriti was the first book written on the law in the world, which deals with social and moral conduct of a person. Manusmriti falls in the Smriti category of Hindu scriptures and hence, it is less authoritative than Vedas and Upanishadas. It is also the most defamed Hindu book as Dr. Ambedkar had burnt the copies of it as a protest against Hindu caste system. To date, many people look at it as a symbol of caste system. The evangelists use this book as a weapon against Hinduism to provoke dalits.
Manusmriti was written in post-Vedic period after the fall of Mauryan Empire but they are based on Dharma Sutras written in Vedic period. The sages who wrote it modified Dharma Sutras to fit the era of rigid caste system prevailed during post-Vedic period.
Manusmriti is also known as Dharmasutras of Manava. There are other Smritis also like Narad Smriti, Yajnavalkya Smriti, Brihaspati Smriti, etc. but Manusmriti is the most famous of them all.
Manusmriti does not talk about caste but it talks about Varna viz. Brahmin, Kshatriya, Vaisya, and Shudra. There is no mention of Varna system according to birth but it advocates the system based on the qualities of a person. If a Shudra possesses qualities of a Brahmin, he could be promoted as a Brahmin but if a Brahmin does not possess qualities required, then he could be demoted as a Shudra.
There are 12 chapters in Manusmriti. It was translated into English by George Buhler. You can read Manusmriti online by following the links given below: