The Akhbaris are the adherents of the ahadith whom the Shia call Akhbaris. They do not believe in Ijtihad and only follow the ahadith. On the other side there is a great group of Muslim scholars known as the Osolioon, they believe that the rulings of Islam should be drawn from it's sources, meaning the Quran, the Sunnah, the Intellect and Ijma’, they also employ the principles of Osol Fiqh like the principles of Bara’ah and Istishab.
There are differences between these two groups, the Osolioon believe in Ijtihad and say that every individual should either be a Mujtahid or follow one (taqlid), they did not allow the following of a dead Mujtahid unless another Mujtahid permits it, they also believe that not all ahadith of the four major fiqh hadith sources of the Shia are authentic. But the Akhbaris dismiss Ijtihad, and argue that death or life does not matter when referring to a Mujtahid to learn the rulings of Islam and consider all of the hadiths recorded in the four major hadith sources as authentic because of the fact that the compilers of these four would put aside any unauthentic hadiths and not have them recorded in their books, while the Osolis still analyze the authenticity of each hadith separately and say that just because these sources have cited a hadith doesn’t mean that it is necessarily authentic.
The Akbarioon and the Osolioon are two groups among the twelver Shias that use different methods to reach the rulings of Islam.
1- The Akhbaris: The adherents of ahadith, the Shia call them Akhbaris. They do not believe in Ijtihad and only follow the ahadith to reach Islam's rulings. It seems that the first person to cause this division among Shia Jurists was Molla Mohammad Amin Ibn Mohammad Sharif Astarabadi (1033 a.h.). Apparently he is the founder of the Akhbari sect and the first among Muslim scholars to pave the way for the criticism and reproach of the mujtahids. In his book "Fawa’idul-Madinah" he has strongly blamed the Mujtahids and accused them of ruining true Islam. He believes that the Ijtihad that is common today is very different from the Ijtihad that was done by the first Muslim scholars among the Shia and that it has been greatly altered. He argued that the Quran consists of different types of verses, there are Nasekh, Mansookh, Mohkam and Moteshabeh verses making it very difficult to use the Quran for reaching Islamic rulings and we can never be sure of the results, therefore we must only rely on the ahadith. He would argue that since ijtihad is based totally on speculation and guesswork, it is unacceptable, but since the ahadith have been issued from the imams, who are infallible, they are for sure and entail certainty, unlike ijtihad that doesn’t bring any surety. When choosing between something that brings certainty and one that doesn’t, clearly the first must be chosen.
2- The Osolioon: The Osolioon are the majority of Muslim Shia Jurists that challenge the opinions of the Akhbarioon. They believe that the results of Ijtihad and scrutinizing the sources of Islamic law (the Quran, the Sunnah, the intellect and Ijma'a) are reliable. They employ the principles of Osol Fiqh such as the principles of Bara'at and Istishab in the process of Ijtihad. They also distinguish between the ahadith that are authentic and those that aren’t. They believe that Ijtihad is wajib kifai (wajib upon all unless the needed amount of people carry it out), or in the case of there only being one person who can carry the task out, it is wajib eyni.
The major difference between them relates to the method they use to draw Islamic rulings from there sources:
1- Ijtihad and Taghlid: The Osolioon believed that every person should either be a Mujtahid or should do Taghlid (follow one who is a Mujtahid and act according to his opinions), but the Akhbarioon believed that Muslims should neither practice ijtihad nor follow a mujtahid.
2- Taghlid e Ibtedaei (Following a Mujtahid that has passed away without the permission of a live one): The Osolioon do not consider Taghlide Ibtedaie permissible but the Akhbarioon argue that being alive or dead doesn’t make a difference when referring to an expert in Islamic rulings.
3- The Akhbarioon assumed that all four of the great fiqh hadith sources of the Shia are totally authentic, arguing that those who compiled the records would not narrate the unauthentic ahadith, thus they don’t categorize the ahadith into the four categories of Sahih, Hasan, Mowathagh and Zaief. But the Osolioon challenge these opinions.
4- The Osolis accept certain concepts such as قبح تکلیف بما لا یطاق (obliging one more than his capacity allows being wrong), or قبح عقاب بلا بیان (punishing without showing the way first being wrong) etc. while the Akhbaris don’t accept any of these intellectual principles because they stem from the intellect and not the ahadith.
5- Acting upon the apparent meaning of Quranic verses: The Akhbarioon believe that Muslims can't and shouldn’t act according to the apparent meaning of Quranic verses unless there are Ahadith that explain the meaning of the verse, but the Osolioon believe that acting upon the apparent meaning of the verses is necessary even if there aren't any ahadith on the meaning of that particular verse (as long as the apparent meaning of the verse is known). For many years there were conflicts and disagreements between the two groups and little by little the Osolioon pushed the Akhbarioon aside, but it was not until the time of Agha Mohammad Baqir Vahid Behbahani (one of the great Osoli scholars) that they were completely defeated. Today Akhbaries are a very small minority among Shia Jurists.