Very nice article by Mufti Ahmad Yaar Khan Naeemi (Alaihi Rahmah). Extracted from 'JAA’AL HAQQ WA ZAHAQAL BAATIL' There are two categories of answerable (mukallaf) Muslims. 1. Mujtahid (One who is qualified in Ijtihad) - A person who so much of knowledgeable ability and understanding that he is able to understand Quranic injunctions, the purpose of the Kalaam (Divine Scripture), is able to extract rules fro it, has complete knowledge over the applicable (naasikh) and inapplicable (mansookh), exceptional linguistic command over Arabic and is well versed in all Quranic verses and Ahadith of laws. Besides these qualities, he must be intelligent and have a comprehending mind. Tasfeeraate-Ahmadia, etc. 2. Ghair-Mujtahid / Muqallid - A person who has not reached such a level.Taqleed is necessary for a Ghair-Mujtahid and is not allowed for a Mujtahid. There are six types of Mujtahids, 1. Mujtahid fi'sh-Shar'i - Those scholars who make the rules (qawaaid) to be followed for ijtihaad, eg. the Four Imaams, ie. Abu Hanifa, Shafee, Maalik and Ahmad ibn Hanbal (RadiAllahu Anhum) . http://www.gatewaytomadina.org/index.php?option=com_content&view=article&id=36n-whom-is-taqleed-binding-obligatory&catid=13:quran-a-ahadith&Itemid=2 Link has the full article.