An assyrtiko is a Greek grape variety and the white wine which is the product of this grape.
Spanakopita, a savory spinach and feta pastry, pairs well with a variety of beverages. A crisp white wine, such as a Sauvignon Blanc or a dry Greek wine like Assyrtiko, complements the flavors nicely. For a non-alcoholic option, consider sparkling water with a splash of lemon or herbal iced tea, which can refresh and balance the richness of the dish.
Retsina, which can only be properly applied to wines made in Greece and parts of Cyprus, is made of Savatiano or Athiri grapes (occasionally with Assyrtiko, Rhoditis, or other local grape varieties added as well). The grapes are juiced and the juice allowed to ferment, but during fermentation small pieces of Aleppo Pine resin are added. When the wine is put into bottles, the pieces of resin remain with the parts of wine not bottled. So, the simple answer is that retsina is made from Greek or Cyprian grapes whose juice has been allowed to ferment in the presence of Aleppo Pine resin.