El Cantar de Mio Cid (or El Poema de Mio Cid, literally The Song of my Cid), also known in English as The Poem of the Cid, is the oldest preserved Castilian epic poem (epopeya). Based on a true story, it tells of the Castilian hero El Cid, and takes place during the Reconquista, or reconquest of Spain from the Moors.
The Spanish medievalist Ramón Menéndez Pidal included the “Cantar de Mio Cid” in the popular tradition he termed the mester de juglaría. Mester de juglaría refers to the medieval tradition according to which popular poems were passed down from generation to generation, being changed in the process. These poems were meant to be performed in public by minstrels (or juglares), who each performed the traditional composition differently according to the performance context—sometimes adding their own twists to the epic poems they told, or abbreviating it according to the situation. (Source Wikipedia)