Metta -- loving kindness -- is one of the "Four Immeasurables" or Four Divine States of Buddhism. These are mental states or qualities cultivated by Buddhist practice. The other three are compassion, sympathetic joy, and equanimity.
Metta is sometimes translated as "compassion," but the Pali language makes a distinction between metta and karuna, which also means "compassion." Karuna connotes active sympathy and gentle affection, a willingness to bear the pain of others, and possibly pity.
Metta is a benevolence toward all beings that is free of selfish attachment. By practicing metta, a Buddhist overcomes anger, ill will, hatred and aversion.
The Metta Sutta is sometimes called the Karaniya Metta Sutta. It is from a part of the Tripitaka called the Sutta Nipata, which is in the Sutra-pitaka, or Sutra Basket, of the Tripitaka. Monks of the Theravada school frequently chant the Metta Sutta.
The Chenrezig Project has an attractive PDF presentation of the sutta to print out and display.
The Theravada website Access to Insight provides a number of translations, including this one by noted scholar Thanissaro Bhikkhu. This is just a small part:
As a mother would risk her life
to protect her child, her only child,
even so should one cultivate a limitless heart
with regard to all beings.