Metta

Metta (Pali) is a benevolence toward all beings, without discrimination, that is free of selfish attachment. It is a strong, sincere wish for the happiness of all beings. 

The Metta Suttacompares metta to the love of a mother, who would give her life for her children.

Metta is often translated as "compassion," which is not inaccurate. However, Pali makes a distinction between metta and karuna, which also means "compassion." Karuna refers to active sympathy and gentle affection, a willingness to bear the pain of others, and possibly pity.

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.

Bodhipaksa

What the Buddha said about metta

Bhikkhus, whatever kinds of worldly merit there are, all are not worth one 16th part of the release of mind by universal friendliness; in shining, glowing, beaming & radiance the release of mind by universal friendliness far excels & surpasses them all.
Itivuttaka 27

As a mother even with own life protects her only child, so should one cultivate immeasurable loving-kindness towards all living beings.
The Metta Sutta

He who both day and night takes delight in harmlessness sharing love with all that live, finds enmity with none.
Samyutta Nikaya. I, 208

What are the eleven advantages of Metta ?

• One sleeps Happy!
• One wakes Happy!
• One dreams no evil dreams!
• One is liked and loved by all human beings!
• One is liked and loved by all non-human beings too!
• One is Guarded & Protected by the divine Devas!
• One cannot be Harmed by Fire, Poison, or Weapons!
• One swiftly Attains the Concentration of Absorption!
• One’s appearance becomes Serene, Calm, & Composed!
• One dies without Confusion, Bewilderment, or Panic!
• One reappears after death on the Brahma level if one has penetrated to no higher level in this very life.
• Aghatavinaya Sutta (Anguttara Nikaya, 5.161)
  For one who mindfully develops
  Boundless loving-kindness
  Seeing the destruction of clinging,
  The fetters are worn away.
  If with an uncorrupted mind
  He pervades just one being
  With loving kindly thoughts,
  He makes some merit thereby.
  But a noble one produces
  An abundance of merit
  By having a compassionate mind
  Towards all living beings.
  Those royal seers who conquered
  The earth crowded with beings
  Went about performing sacrifices:
  The horse sacrifice, the man sacrifice,
  The water rites, the soma sacrifice,
  And that called “the Unobstructed.”
  But these do not share even a sixteenth part
  Of a well cultivated mind of love,
  Just as the entire starry host
  Is dimmed by the moon’s radiance.
  One who does not kill
  Nor cause others to kill,
  Who does not conquer
  Nor cause others to conquer,
  Kindly towards all beings —
  He has enmity for none.
  (Itivuttaka, 1.27)