Why a Blessing?
Few activities are as instinctive as eating: it is a basic bodily need, requires no thought or advance preparation, and is, largely, for us today, readily available. And yet the Torah demands from us that before putting anything in our mouths, we pause for a moment, recite a few words to ourselves, and only then proceed with our eating. And with these words, this simple blessing, we have transformed the most prosaic of acts into something holy.
How? By acknowledging G-d as the source of all sustenance, recognizing that the earth and its bounties belong to Him, and expressing our gratitude to Him for providing it for us. All this with the words: “Blessed are You, Lord our G-d, King of the universe…”
Main Link:
http://www.chabad.org/library/article_cdo/aid/90550/jewish/Why-a-Blessing.htm
Basic Rules:
Before:
http://www.chabad.org/library/article_cdo/aid/91119/jewish/Laws-of-the-Blessings-Before-Food.htm
After:
Food Blessing Guide:
http://www.chabad.org/library/article_cdo/aid/278538/jewish/Basic-Blessings-on-Food-Guide.htm