Sahabi (Companion) is anyone that met the Prophet (pbuh) while believing in him, and dying upon that condition.
The Companions (Sahabah) of the Prophet (pbuh) are the best from all those that accompanied the prophets, due to the statement of Allah's Messenger (pbuh): "The best of people are my Companions." [1]
The best among the Sahabah are the Muhajirun (those who performed the migration from Makkah to Madinah), due to their combining between migrating and giving support (in Allah's Cause). Then after them, are the Ansar.
The best among the Muhajirun are the four righteous Khalifahs: Abu Bakr, Umar, Uthman and Ali.
Abu Bakr - He is "As-Siddiq" 'Abdullah bin Uthman bin Amir from the Tribe of Taym bin Marrah bin Kaab. He was the first male to believe in the Messenger of Allah (pbuh). He was his companion in the migration (Hijrah) and his deputy during times of prayer and Hajj. He was his Khalifah (designated successor) for his nation. Five of those who were given the glad tidings of Paradise accepted Islam through his hands.
They are Uthman, Az-Zubair, Talha, Abd-ur-Rahman bin Awf and Saad bin Abi Waqqas. He died in Jumadal-Al-Akhirah, in the 13th year of Hijrah, when he was 63 years old. The above five along with Abu Bakr, Ali bin Abi Talib and Zayd bin Harithah form the eight that were the first of people to accept Islam. Ibn Ishâq stated this and what he meant by it, was that they were the first from amongst the males, after (the advent of) the Message.