Feelings of serenity, hope, pride, etc. Positive emotion falls into three categories: about the past (satisfaction), about the present (pleasures and gratifications), and about the future (optimism).
The outward display of positive emotion is called positive affect. Women tend to experience more negative and more positive emotions than men.

Benefits

Health: Positive affect is correlated with better health decisions, lower blood pressure, and a better immune system. Positive emotions can help you recover from the physiological effects of negative emotions faster. In one study, nuns who expressed more positive emotion in their biography when entering the order lived longer.

Happiness: Another study found that women smiling genuinely (Duchenne smile) in their yearbook photos end up happier later in life.

Anti-depression: Frequent positive emotions reduce depressive symptoms

Other traits: Happy moods make people more productive, active, friendly, helpful, resilient, and creative. We perceive a day with more positive emotion as more meaningful. Positive emotion also expands our intellectual, physical, and social resources: we’re more open and creative, and people like us better.

Sources: Sonja Lyubomirsky, The How of Happiness; Martin Seligman, Authentic Happiness

Who studies it

Roy Baumeister, Fred Bryant, Robert Emmons, Barbara Fredrickson, Shelly Gable, Todd Kashdan, Dacher Keltner, Tali Sharot

Books

Fred Bryant and Joseph Veroff, Savoring: A New Model of Positive Experience (2006)

Robert Emmons, Gratitude Works!: A 21-Day Program for Creating Emotional Prosperity (2013), THANKS!: How Practicing Gratitude Can Make You Happier (2008), Words of Gratitude for Mind, Body, and Soul (2001)

Barbara Fredrickson, Love 2.0: Finding Happiness and Health in Moments of Connection (2013)

Dacher Keltner, Understanding Emotions (2013)

Terry Paulson, The Optimism Advantage: 50 Simple Truths to Transform Your Attitudes and Actions into Results (2010)

Martin Seligman, Learned Optimism: How to Change Your Mind and Your Life (1991)

Tali Sharot, The Optimism Bias: A Tour of the Irrationally Positive Brain (2011)

Subcategories

Satisfaction, pleasure, gratification, optimism, joy, gratitude, serenity, interest, hope, pride, amusement, inspiration, awe, love