"And you shall eat, and be sated, and bless the L-rd your G-d" (Deuteronomy 8:10). In addition to the before-eating blessings instituted by the sages, we have a biblically-mandated obligation to thank and bless G-d after eating, expressing our gratitude to the One who "nourishes the entire world with His goodness, with grace, with benevolence and with compassion" (from "Grace After Meals").
Grace After Meals consists of four primary blessings -- the first composed by Moses when the manna came down from heaven in the desert, the second by Joshua when the Children of Israel ate from the first harvest after entering the Holy Land, the third by Kings David and Solomon, and the fourth by the Sages in mishnaic times.
The full "Grace" is recited only after partaking of a meal that includes bread, and covers everything eaten during the meal.
See "Readings and Resources" below for the full text.