Simply stated:
Tonglen ('giving and taking') is a mediation to take on some level of suffering of another person. On in the in breath one imagines the suffering or pain being taken on, and on the out breath one imagines sending love and compassion.
Metta means 'loving kindness'. When we say "May all beings be happy! May all beings be joyful! May all beings be at peace!" this is a practice of metta. However there are entire meditations of metta. First you send love to yourself, then to a friend, then to a neutral person, then to a difficult person, then to larger and larger populations of people until one is sending love out to the whole universe. You can spend about 10 minutes at each stage.