Well, as you might already know, Islam is based on the Qur'an and the Ahadeeth.
The Qur'an is the word of God which was revealed over 1400 years ago to Prophet Muhammad (SallAllahuAlayhiWaSallam) by God through Arch-angel Jibreel (Gabriel - peace be upon him). Whereas Ahadeeth (singular - Hadith) has been very subtly defined by wikipedia as "The term Hadīth is used to denote a saying, act or tacit approval either validly or invalidly ascribed to the Islamic prophet Muhammad (SallAllahuAlayhiWaSallam)". So ahadeeth are all directly related to the Prophet. The acts that he commanded, did and approved are recorded in the ahadeeth collections. As far as I know, there have been 10 collections of Ahadeeth (there might be more) but only 6 of them contain authentic ahadeeth in majority. The six collections are:
- Sahih Bukhari
- Sahih Muslim
- Sunan Ibn Majah
- Sunan An Nasai
- Sunan Abi Dawood
- Sunan Tirmidhi
The Qur'an was prepared and completed during the lifetime of the Prophet as he would communicate the revealed verses to his companions and they would write it down and maintain a record. Whereas the Ahadeeth collections came into picture after more than a century of Prophet Muhammad's (SallAllahuAlayhiWaSallam) death. The companions refrained from maintaining a record of the Ahadeeth during the time of the prophet because of the fear that it might get mixed with the Qur'an and create unnecessary confusion. For all those years, the Ahadeeth were passed on only through memory.
Eventually, after century(s) the task of publishing them in written form started and scholars like Imam Bukhari (the collector of Sahih Bukhari) started the search of Ahadeeth. As they were passed on from people to people and generation to generation, every hadeeth had a long chain of narrators. And the authencity of a hadeeth is based on the credibility of the narrators. And as per the credibility, each Hadeeth is labelled such as Saheeh (Authentic), mawdouh (fabricated), daeef/zaeef (weak), Hasan (good) and others.
Popularly, the Qur'an can be found in every Muslim home and every Muslim gathering place but not so the collection of Ahadeeth. The Qur'an can be studied and understood without the help of a teacher to a considerable extent. The Ahadeeth need special teachings. In some cases, the narrations are short, in some are long, in some it is incomplete, in some it is pointless and other such scenarios. Hence, it is difficult to study them all by yourself. The ahadeeth are so because the narrators narrated them that way and to learn them well with their precise meaning, you have to search other Ahadeeth and find the context mentioned to understand the meaning behind it. If you find Sahih Muslim or Sahih Bukhari and you get enthusiastic and start reading it, there is a high chance that you won't understand a lot of it. Many hadeeth need further research and study to realize their aimed message. Although, there are plenty of narrations which are direct to the point and don't need any further search such as the one in my signature. It was realized only later (after the assassinations of Umar and Usman - RadhiAllahu Anhu) that it was a prophecy.
It was narrated that Anas ibn Maalik (may Allaah be pleased with him) said:
The Prophet (blessings and peace of Allaah be upon him) climbed up Uhud, accompanied by Abu Bakr, ‘Umar and ‘Uthmaan, and the mountain shook with them. He struck it with his foot and said: “Stand firm, O Uhud, for there is no one on you but a Prophet or a Siddeeq or two martyrs.”
Narrated by al-Bukhaari (3483)
I hope this answers all your questions.
P.s. - welcome to the forum
Also go through these links:
Conditions of a saheeh (sound) hadeeth
The authors of the Six Books
Definition of a mursal hadeeth
Definition of a ghareeb hadeeth
Who compiled saheeh ahaadeeth apart from al-Bukhaari and Muslim?
How can we tell the saheeh ahaadeeth from the fabricated ones?