What exactly is a hadith? It’s a saying of the Prophet (صلي الله عليه وسلم). The Science of Hadith defines a hadeeth as “whatever is attributed to the Prophet, in terms of sayings, actions, approvals, and physical and character descriptions.” 
Ahadith are an important part of Islam. They complement and the Qur’an, and provide us with numerous benefits and different types of information, including:
- Laws not directly stated in the Qur’an (eg. prohibition of visiting fortune-tellers)
- Details of things mentioned in the Qur’an (eg. how to make wudoo)
- A look into the lifetime of the Prophet (صلي الله عليه وسلم)
- Tafseer (explaination) of the Quran (eg. the story of the People of the Ditch)
Some people deny ahadith, and say “we don’t need no hadiths! We can just use Qur’an!” In response, refer them to this verse:
Translation: O you who believe! Obey Allâh, and obey the Messenger (Muhammad) and render not vain your deeds. [Surah Muhammad, 47:33]
Or, if it suits your da’wa a bit better, ask them: “how do you know how to pray?” It’s not in the Qur’an–only a few details here and there. The same goes for wudoo! (Further details are available here.)
While ahadith can be classified in many ways, it’s essential that all Muslims understand the following five:
- Saheeh (linguistically, “sound”): those ahadith that are certified authentic.
- Hassan (linguistically, “good”): those ahadith that are lower in status then saheeh. They are usually da’eef hadith that are upgraded to Hassan due to authentic backing.
- Da’eef (linguististically, “weak”) those ahadith that are likely not true and are generally not used in certain topics, such as ‘aqeedah. They can still be used in other fields, such as akhlaaq.
- Mawdoo’: Fabricated ahadith that are definitely not true–someone invented them and attributed them falsely to the Prophet.
- Qudsi: (linguistically, “pure”): Hadith where the Prophet narrates or paraphrases something from Allah (سبحانه وتعالى).
That includes a brief introduction to ahadith. Please post any questions/clarifications in the comments area. Jazakumullahu khayran!
 Hamid, Abdool. “Mustalahul-Hadeeth”: the Science of Hadeeth Methodology. Toronto: The Islaamic Institute of Toronto. 2.