Emotion is defined as any mental experience with high intensity and high hedonic content. The existence and nature of emotions in non-human animals are believed to be correlated with those of humans and to have evolved from the same mechanisms. Charles Darwin was one of the first scientists to write about the subject, and his observational (and sometimes anecdotal) approach has since developed into a more robust, hypothesis-driven, scientific approach.
Discrete emotion theory is the claim that there is a small number of core emotions. For example, Silvan Tomkins (1962, 1963) concluded that there are nine basic affects which correspond with what we come to know as emotions: interest, enjoyment, surprise, distress, fear, anger, shame, dissmell (reaction to bad smell) and disgust. More recently, Carroll Izard at the University of Delaware factor analytically delineated 12 discrete emotions labeled: Interest, Joy, Surprise, Sadness, Anger, Disgust, Contempt, Self-Hostility, Fear, Shame, Shyness, and Guilt (as measured via his Differential Emotions Scale or DES-IV).