Verily, those who give Bay'ah (pledge) to you (O Muhammad S) they are giving Ba'iah (pledge) to Allah. The Hand of Allah is over their hands. Then whoever breaks his pledge, breaks it only to his own harm; and whoever fulfils what he has convenanted with Allah, He will bestow on him a great reward.
Surah (Chapter) Al-Fath (The Victory) The Qur-an 48:10

A Sahabi (Companion of Muhammad S) is a Muslim who had met the Messenger of Allah and died as a Muslim. The Sahabah (Companions of Muhammad S) were exceptional Muslims, who were willing to any thing for the cause of Allah (God) and His final Rasul (Messenger) even at the cost of their lives.
They spent their lives strictly in accordance to the Islamic code taught and demonstrated to them by the last Rasul (Messenger) of Allah, Muhammad S. They sacrificed everything for the sake of Allah (God) to follow the Sunnah (Traditions of Muhammad S).

And the foremost who embraced Islam of the Muhajirun (those who  migrated from Makkah to Al-Madinah) and the Ansar (the citizens of
Al-Madinag who helped and gave aid to the Muhajirun) and also those who followed them exactly (in Faith). Allah is well-pleased with them as they are well-pleased with Him. He has prepared for the Gardens under which rivers flow (Paradise), to dwell therein forever. That is the supreme success.
Surah (Chapter) at-Taubah (The Repentance) Qur-an 9:100

The Sahabah (Companions of Muhammad S) strived hard to stay close to the Rasul (Messenger) of Allah, Muhammad S to learn Islam. They faithfully transmitted the Sunnah (Traditions of Muhammad S) to the next generation. Their self-less hard work is evident in the one and a quarter billion strong Muslim Ummah (Community), which try to faithfully follow the Sunnah (Traditions of Muhammad S).

Muslims give a lot of respect to Sahabah (Companions of Muhammad S) and recite a Du'a (Invocation) upon taking their names.
Radi Allahu 'Anhu for male Sahabi and Radi Allahu 'Anha for female Sahabiyah. It means 'May Allah be pleased with him / her'.

In our writings we are writing R for Radi Allahu 'Anhu or Radi Allahu 'Anha.