Ahlul Bayt (Ahl al-Bayt) is a phrase meaning People of the House, or family. In the Islamic tradition it refers to the household of Prophet Muhammad (pbuh).
Ahlul Bayt (Ahle Bait) in simple terms is to put trust of guidance to Aal-e-Muhammad [descendents of Prophet Muhammad (pbuh)], through Fatima Zahra (pbuh) and Imam Ali (pbuh) and their descendents. Bayt is to give the Oath of Allegiance to Imam Ali (pbuh) and accept his guidance.
Ahlul Bayt (Ahl al-Bayt) or household members of Prophet Muhammad (pbuh) refers to his daughter Fatima Zahra (pbuh), his successor and son-in-law, Imam Ali (pbuh), their two sons Imam Hasan (pbuh) and Imam Hussain (pbuh), and the nine Imams from the lineage of Imam Hussain (pbuh).
Ahlul Bayt (Ahl-e-Bait) in Noble Qur'an
The Purified Ones: The Highest degree of purity means to be kept constantly away from all the causes of impurity. This is termed as the state of infallibility in knowledge, character and action. It could have been applied generally to the whole mankind who are keeping aloof from all the impurities as the word of Allah (SWT) commands; but, Allah (SWT) expressively has confined His order to certain group of individuals by excluding the rest of the mankind from it in His divine will by declaring Ahlul Bayt (Ahl al-Bayt) as the persons purified by Him to be constantly in touch with the Noble Qur'an in its original, hidden, well protected, exalted and purified form. It was Allah's (SWT) wish to remove all blemishes from them, as mentioned in the Verse of Purity (Ayat Al-Tathir) in the Noble Qur'an:
The Verse of Purity (Ayat Al-Tathir): "... Allah only desires to keep away the uncleanness from you, O people of the House! And to purify you a (thorough) purifying." Noble Qur'an (33:33)
The above verse from Noble Qur'an, is known as Ayat Al-Tathir, refers to the members of the household of Prophet Muhammad (pbuh), which include exclusively Prophet Muhammad, Imam Ali, Fatima Zahra, Imam Hasan, and Imam Hussain (peace be upon them all). Hence Prophet Muhammad's (pbuh) other offspring, wives, sons of paternal uncles, and dwellers of his house are not to be called as Prophet Muhammad's Ahlul Bayt (Ahl al-Bayt).
The argument is based on the genuine and authentic traditions narrated by Sahaba, companions of Prophet Muhammad (pbuh) recorded in both the Sunni and Shia sources.
source : http://www.ezsoftech.com/stories/ahlulbayt.asp
