:salamext:
The first thing we must understand is that the Qur'an
as well as the Sunnah are both revelation from Allaah :swt:. Allaah :swt: revealed the Qur'an to the Prophet :saws: and commanded him to explain it to the people. The Sunnah of the Prophet :saws: is the practical example of the implementation of the Qur'anic guidance, the Wisdom taught to the Prophet :saws: along with the Scripture. This means that these two can never contradict one another, rather they complement one another. Neither the Qur'an nor the Sunnah can be understood correctly without recourse to the other. Allaah :swt: tells us in the Qur'an that the Messenger :saws: is to be obeyed. He :swt: is not only talking to that honourable group of Muslims that accompanied the Prophet :saws: during his lifetime, rather He is speaking to all of the Muslims that will come up until the Last Day.
In a similar way to how the Qur'an was preserved, Allaah :swt: preserved the Sunnah by enabling the Companions and those after them (may Allah be pleased with them) to memorise, write down and pass on the statements of the Messenger :saws: and the descriptions of his way. All of this happened in various stages, beginning with the writing of hadeeth on various parchments without any particular format, until they were compiled into collections on a wide and systematic scale. Then there arose scholars who undertook the job of critical research of the hadeeth that had been narrated and compiled, and they also grouped hadeeth according to their topic. Many scholars journeyed tirelessly to collect hundreds of thousands of narrations and distinguish the true words of precious wisdom of their Messenger :saws: from those corrupted by weak memories and from forgeries by unscrupulous liars. All of this was achieved through precise attention to the words narrated and detailed familiarity with the biographies of the thousands of reporters of Hadith. The following sheds light on this last sentence, concerning how Hadith were reported from the Prophet :saws::
The Muslims are agreed that the Sunnah of the Prophet Muhammad (may Allah bless him and grant him peace) is the second of the two revealed fundamental sources of Islam, after the Glorious Qur'an. The authentic Sunnah is contained within the vast body of Hadith literature. [1]
A hadith (pl. ahadith) is composed of two parts: the matn (text) and the isnad (chain of reporters). A text may seem to be logical and reasonable but it needs an authentic isnad with reliable reporters to be acceptable; 'Abdullah b. al-Mubarak (d. 181 AH), one of the illustrious teachers of Imam al-Bukhari, said, "The isnad is part of the religion: had it not been for the isnad, whoever wished to would have said whatever he liked." [2]
During the lifetime of the Prophet (may Allah bless him and grant him peace) and after his death, his Companions (Sahabah) used to refer to him directly, when quoting his sayings. The Successors (Tabi'un) followed suit; some of them used to quote the Prophet (may Allah bless him and grant him peace) through the Companions while others would omit the intermediate authority - such a hadith was later known as mursal. It was found that the missing link between the Successor and the Prophet (may Allah bless him and grant him peace) might be one person, i.e. a Companion, or two people, the extra person being an older Successor who heard the hadith from the Companion. This is an example of how the need for the verification of each isnad arose; Imam Malik (d. 179) said, "The first one to utilise the isnad was Ibn Shihab al- Zuhri" (d. 124).[3] The other more important reason was the deliberate fabrication of ahadith by various sects which appeared amongst the Muslims, in order to support their views (see later, under discussion of maudu' ahadith) [...]
A brief history of Mustalah al-Hadith
As time passed, more reporters were involved in each isnad, and so the situation demanded strict discipline in the acceptance of ahadith; the rules regulating this discipline are known as Mustalah al-Hadith (the Classification of Hadith)...
http://www.islamicboard.com/hadeeth/134278656-introduction-sciences-hadith-shaykh-suhayb-hasan-hafidhahullaah.html
This science of Hadith became a very detailed science, whereby hadeeth were graded according to various criteria, indicating their level of authenticity. So for example, some of the criteria for a hadeeth to be authentic includes having an uninterrupted chain of narration, uprightness of the narrators, and veracity and accuracy of the narrators. If you continue reading the thread from which the above quote has been taken, you will gain an appreciation for how detailed and meticulous this science of Hadith is.
Regarding the names of the collections, at least some of them represent a particular stage or methodology of compiling the Hadith. Among the most important collections are the famous six books of hadeeth, their authors being Imam al-Bukhari, Imam Muslim, Imam Abu Dawood, Imam al-Tirmidhi, Imam al-Nasaa’i and Imaam Ibn Maajah, may Allaah :swt: have mercy on them all. And among these, the works of Imams Al-Bukhari and Muslim occupy the highest status in terms of authenticity, because these scholars focused on including only those Hadith of the highest degree of authenticity and whose narrators were known for their formidable memory and accuracy. Other scholars of Hadeeth did not stipulate these conditions. Al-Bukhari devoted more than a quarter of his life to his compilation and is said to have questioned more than a thousand hadeeth scholars living in various different regions. His compilation is said to have been heard by 90,000 of his students. Al-Bukhari memorised not only the hadeeths in the books of the scholars but also the biographies of all the narrators in their chains (including their dates and places of birth and death). He used to seek aid in prayer before recording every tradition, and weighed every word he wrote with scrupulous exactitude. He selected 9,082 out of some 600,000 narrations of hadeeth. If repetitions are excluded, the actual number of hadeeths goes down to 2,062. It is generally considered by the Muslims as an authority second only to the Qur'an.
It is indeed fascinating to learn about the science of Hadith. We have some useful threads in this section which go into details regarding the many evidences for the authority of the Sunnah, details regarding the classification of hadeeth and other aspects. As brother Hulk mentioned, the best way is to find a good teacher who will guide you through it in a structured manner.
Allaah :swt: knows best. May He :swt: guide us to be amongst the People of the Sunnah, those who venerate it in their hearts and through their actions, Aameen.