Pastoral literature is literature about idealized rural characters living idyllically simple, rural lives (traditionally: shepherds who write love poetry to pass the time). This is pastoral: http://www.youtube.com/watch?v=fW1gMVw_LtI
As for “cold” – — pastoral scenes are beautiful, in part, because they’re completely removed from real life. The scene on the urn is both frozen in place (“cold”) and frozen outside time.
To help improve the meaning of these lyrics, visit "Ode on a Grecian Urn" by John Keats and leave a comment on the lyrics box