Manu, Parasara, Yajnavalkya, Gautama, Harita, Yama, Visnu, Sankha, Likhita, Brhaspati, Daksa, Angiras, Pracetas, Samvarta, Acanas, Atri, Apastamba and Satatapa are the eighteen sages who mastered the Vedas with their superhuman power and derived the Smrtis from them. Their works are known after them like Manusmrti, Yajnavalkya-smrti, Parasara-Smrti and so on, and they contain all that we need to know about all the dharmas to be adhered to and all the rituals to be performed during our entire life.
Apart from these eighteen , there are eighteen subsidiary Smrtis called Upasmrtis. It is customary to include the Bhagavadgita among the Smrtis.
What we find in one Smrti may not be found in the other. There may also be differences between one Smrti and another. These give rise to doubts which are sought to be cleared by works called "Dharmasastra Nibandhanas".
There are some Smrtis which do not contain instructions with regard to all observances. For instance, some do not mention sandhyavandana. The reason must be it is such a common rite that everybody is expected to know it. Then some omit the sraddha ceremony and some others are silent on various types of "pollution" (for instance, that due to the birth of a child in the family or death of a relative). Certain matters are taken for granted. After all, we do not have to be told about how to breathe or eat.
The nibhandanas do not leave out any rite or dharma. Differences between various Smrtis are sought to be reconciled in them.
Each region follows its own nibhandhana. In the North, it is the one authored by Kasinatha Upadhyaya. In Maharastra, it is the Mitaksara: it has the force of law and is accepted as such by the law courts. Nirnayasindhu by Kamalakara Bhatta is also accepted as an authority there. In the South, Vaidyanatha-Diksitiyam by Vaidyanatha Diksita is followed. These are the important authorities for householders. Sannyasins follow Visvesvara-samhita. In Tamil Nadu the Dharmasastra means the Vaidyanatha-Diksitiyam. The nibandhana has been translated into Tamil.
The Dharmasastras are not as difficult to follow as the Vedas and can be understood with a little knowledge of Sanskrit.
For a general background, please see here