Lambs are typically weaned to a diet of grass from 1-3 months old and have more tender and delicately-flavored meat. Muttons are older animals that have had a good couple of years grazing on pasture (and preferably wild herbs and mixed grasses) and are full of flavor and taste wonderful after a long, gentle cook. It can take strong spices for more exotic dishes such as mutton biryani but also tastes wonderful roasted with Mediterranean herbs. If you want a fall-off-the-bone slow roast, mutton is for you. You can find more information about it here.