Cyrus the Great was a grandson to a king of the Medes, who were then the rulers of the tribes that would later become Persia. His grandfather wanted him dead to prevent him from taking the throne. However, he was saved by a shepherd and his wife, and brought up. After this, as a young man, he met his grandfather and was identified as Cyrus. He became ruler of the Persians, who were then a province of the Median Empire. He revolted not long after, and, killing his grandfather, he became ruler of all the provinces that belonged to the Medes, but was also the first official ‘Persian Emperor’. The empire was also officially ‘The Persian Empire’ as well. One of the things King Cyrus was known for is letting the Jews return from Babylon, where they had been exiled; for, Cyrus had conquered Babylon. The 470 years between when Cyrus let the Jews return and when Rome conquered Judea were free and happy days for the Jews. Cyrus was also known for expanding off of the Median empire to control all of Asia Minor (and, unless it was Darius the Great who did it, expanded much farther east into parts of India).
After Cyrus died, his son, Cambyses II came to the throne for a short time. He died marching against Egypt. For a short time an imposter reigned, before he was killed by Darius the Great, a son-in-law to Cyrus. Darius was also distantly related by blood to Cyrus, and this was how he laid claim to the throne.