This is a famous and beautiful piece by Johann Sebastian Bach. Bach was the most prolific composer in baroque music. He wrote hundreds of pieces of music in every type except opera.
Bach was not famous during his lifetime. He was a composer and choir director at a church for most of his life. His manuscripts, (written music) were discovered long after he died and people realized how incredible his music was and started to play it.
Since then, Bach is generally considered the greatest composer of all time.