Grass-Fed means the cow was fed grass during it's life. Notice it does not say, "the cow was fed grass during it's ENTIRE life." A cow can be considered grass-fed even it was only on grass for 60% of it's life. Most cattle are on grass for the beginning part of their lives and then taken to a feedlot and fattened up with grains and corn for the remainder of their lives. Giving the cows anything but grass drastically changes the taste and the nutritional value, even if it's just for a few days.
At the 2S Ranch our beef is 100% grass-fed and grass-finished. Grass-finished means that the cattle strictly eat grass even at the end of their lives. No corn or grains are ever given to our cattle.