There are a few different ways to categorize theories of emotions. The one I will focus on here is the distinction between models in which emotions are considered discrete (basic emotion models) and those in which emotions are considered continuous (dimensional models).

Basic Emotion Models

     "In most basic emotion models, each emotion is caused by a dedicated mechanism (a definable brain circuit or affect program) that produces a coordinated package of experiences, incipient response tendencies, expressive behaviors (e.g., facial expressions), and autonomic and neuroendocrine responses" (Gross & Barrett, 2011). In this way, there is a limited number of basic emotions, each of which is a unique biological state caused by its own neural circuit. The most well known Basic Emotion Model is that of Paul Ekman, who posits the existence of seven basic emotions: anger, contempt, disgust, enjoyment, fear, sadness, and surprise, based mainly on his research on universal facial expressions of emotions. All other emotions are presumed to be made by combinations of these basic emotions.

    Currently, basic models of emotion dominate the field of emotion research, but there are compelling reasons for using a dimensional approach, and they are becoming more popular.

Dimensional Models of Emotion

     Dimensional models of emotions do not posit a small set of discrete emotions, each with its own neural circuit, that are the building blocks of all emotional experience, but instead propose that a spectrum of emotions can be generated by the activities of distributed brain networks, with a small number of underlying neurophysiological systems corresponding to dimensions that can be used to define a conceptual affect space. In this way, different emotions are mapped to different locations in the affect space based on cognitive interpretations of unique combinations of activities of the underlying systems. In dimensional models of emotions, a small number of dimensions can be used to resolve a much larger set of emotions from one another. There are no basic emotions here; all emotions are constructed and vary only by the differential patterns of activation in the systems used to define the affect space. 

     There are several dimensional models of emotion, including that of Watson & Tellegen (shown above), but the most authoritative theory was developed by James A. Russell and is known as the Circumplex Model of Affect. 

REFERENCES

‣ Ekman, Paul (1999). Chapter 3: Basic Emotions. Handbook of Cognition and Emotion. https://www.paulekman.com/wp-content/uploads/2013/07/Basic-Emotions.pdf

‣ Gross, James J. & Barrett, Lisa F. (2011). Emotion Generation and Emotion Regulation: One or Two Depends on Your Point of View. Emot Rev. Jan; 3(1): 8–16.doi: 10.1177/1754073910380974