These animals ate grass their entire lives — no grain, ever. Grass fed beef tends to be leaner with a slightly different flavor profile than grain fed. If you've had it before and liked it, you know. If you haven't, it's worth trying.
Research consistently shows that 100% grass fed beef contains significantly more omega-3 fatty acids than grain-fed beef, along with higher levels of vitamins A and E and more conjugated linoleic acid (CLA). It also tends to be lower in total fat and has a more favorable ratio of omega-6 to omega-3 fatty acids — something nutritionists generally consider a good thing.