A Pastoral poem, or Idyll, is a type of poem that idealizes a simple country life. For this question both: Pastoral poem and Idyll are correct. Although they are not necessarily synonyms, they both describe an idealization of simple country life.
Idyll refers to any poem that describes simple life in nature, while Pastoral usually refers specifically to shepherds, (characters that personify shepherds). The word Pastoral derived from "pastoralism", which refers to the life of shepherds herding livestock in rural areas.
While Pastoral refers to a greater artistic genre, including literature, art and music, Idyll describes poetry or a type of musical composition. An Idyll describes everyday life instead of great endeavors like warfare, heroic actions etc. Idyllic is also an adjective describing an ideal situation. In both Pastoral literature and an Idyll, life in natural settings is idealized and thought of as carefree and happy.