Gil Fronsdal provides an excellent introduction to the four positive qualities of the heart known as the Brahmaviharas. The four qualities, lovingkindness, compassion, appreciative joy, and equanimity are also known as the four immeasurables, because they can be extended to all beings without measure. The article includes a brief discussion of the use of meditation to cultivate those qualities. You can find his article here: The Four Faces of Love: The Brahma Viharas
Sylvia Boorstein discusses the Brahmaviharas and the way they interact with one another, and also the way they can support us in daily life. You can find her article here: Living in the Divine Abodes