In the Buddhist Theravadan tradition, metta means "lovingkindness." In the Pali Canon there are many "suttas" or passages, that deal with the cultivation of virtue, and the proper expression of metta.
A traditional formula for expressing metta to the world is the prayer: "May all beings be free from enmity, affliction and anxiety, and live happily."
Metta meditation, as it is taught these days (to Westerners in particular) can be approached in stages. As it is generally easiest to cherish oneself, the meditator is instructed to direct lovingkindness inwardly. The next, more advanced stage involves radiating love to a good friend; next, to a neutral person, and then to an enemy or difficult person; and finally, to the entire universe.
Learning this practice can take a very long time.