Ansar is the plural form of Naser from the root of Nasr, and means people who help and aid. In the advent of Islam, the residents of Medinah and its outskirts, especially the members of the two tribes of the Aws and Khazraj were called the Ansar, because they had helped the prophet (pbuh) and his followers that had all migrated from Makkah and other places and didnt have anywhere to stay in Medinah. They had a very significant influence in the spread of Islam.
The prophet (pbuh) established a pact of brotherhood between the Ansar and Muhajirin (immigrants from Makkah) upon the arrival of the Muhajirin at Medinah. The first group of Ansar who pledged their beyah (allegiance) to the prophet (pbuh) outside of Makkah were six or seven individuals from the tribe of Khazraj. The Quran has praised the Ansar and so have the prophet (pbuh) and imams.
After the demise of the prophet (pbuh), some of the Ansar would say that the successor to the prophet (pbuh) must be chosen from amongst them, but tension grew amongst the heads of the Aws and Khazraj over who should be chosen, and the support of some of the Ansar regarding Imam Ali (as) being successor was ineffective. These and other factors contributed to the Ansar gradually receding from this matter and to eventually give in to the decision of the council of Saqifah of Bani Saidah which was to give the successorship to the Muhajirin.
After the wealthy and noblemen of Quraysh gained power during Uthmans rule, the dissatisfaction of the Ansar regarding the Muhajirin reached its highest and as a result, they gradually left Medinah to join the Islamic army and the frontier guard.
Some of the Ansar gave in to the circumstances and accepted subordinate positions in the Umayyid government. Of course, there were also individuals amongst the Ansar who were lovers and followers of the Ahlul-Bayt.
 Dairatul-Maarife Tashayyu, Al-Maarif wal-Maarif.