In the Buffy universe, Diana was the Goddess of Love and the Hunt. Amy Madison once invoked her to cast a potent love spell.
Behind the scenes:
Diana is probably based on the eponymous Roman goddess, who was in turn based upon the Greek goddess Artemis. However, in Roman mythology, Diana is not the Goddess of Love, but is the Goddess of the Moon, the Hunt, Wild Nature, and Maidenhood.