Rodrigo Díaz de Vivar was a knight of Castile in the eleventh century; highly talented, he became the armiger regis to Sancho II, despite being only from the lesser nobility. During Sancho's rule, he led several successful campaigns against Alfonso VI and Garcia II (Sancho's brothers) as well as against the Almoravid Muslim kingdom of Andalusia. However, Sancho was murdered and Alfonso took the throne of Castile; thus he fell from favor and was eventually exiled. With nowhere else to go, he took work defending the Muslim kingdom of Zaragoza. Again, he was consistently successful, and Alfonso, facing a powerful and succesful Almoravid, had to give in and beg him to come back. He then went on to become ruler of Valencia, where he was very popular with both Christians and Muslims -- as was perhaps inevitable, because there was no ruler in Spain under whom they could possibly been safer. It is likely in Valencia that he began to be called as-Sayyid, the Lord, which gives him the name under which he has always been known since, El Cid. (In his lifetime, he was probably more often known as El Campeador.) And, indeed, despite Almoravid attempts to unseat him, he remained victorious until his death in 1099. After his death, however, the Almoravids were able to force his family into surrender.
El Cantar de mio Cid is the most famous telling of his legend, written in Old Castilian in the twelfth century. We actually don't know the title of the work -- as it has come down to us, the manuscript is missing its first part. It is, of course, the next fortnightly book. I'll be reading it in the Penguin Classics, translated by Burton Raffel, which has the Castilian and the English in parallel, which should be fun.