Metta is an attitude of recognizing that all sentient beings (that is, all beings that are capable of feeling), can feel good or feel bad, and that all, given the choice, will choose the former over the latter.

Metta is a recognition of the most basic solidarity that we have with others, this sharing of a common aspiration to find fulfillment and escape suffering.

Metta is empathy. It’s the willingness to see the world from another’s point of view: to walk a mile in another person’s shoes.

Metta is the desire that all sentient beings be well, or at least the ones we’re currently thinking about or in contact with. It’s wishing others well.

Metta is friendliness, consideration, kindness, generosity.
Metta is an attitude rather than just a feeling. It’s an attitude of friendliness.

Metta is the basis for compassion. When our Metta meets another’s suffering, then our Metta transforms into compassion.

Metta is the basis for shared joy. When our Metta meets with another’s happiness or good fortune, then it transmutes into an empathetic joyfulness.

Metta is boundless. We can feel Metta for any sentient being, regardless of gender, race, or nationality.

Metta is the most fulfilling emotional state that we can know. It’s the fulfillment of the emotional development of every being.
It’s our inherent potential. To wish another well is to wish that they be in a state of experiencing Metta.

Metta is the answer to almost every problem the world faces today. Money won’t do it. Technology won’t do it. Metta will.

Source: http://www.wildmind.org