The picture above is by Machine Elf 1735 - Own work, Public Domain, https://commons.wikimedia.org/w/index.php?curid=13285286
Some Definitions
Emotion
Emotions can be understood as either states or as processes. When understood as a state (like being angry or afraid), an emotion is a type of mental state that interacts with other mental states and causes certain behaviours.
Understood as a process, it is useful to divide emotion into two parts. The early part of the emotion process is the interval between the perception of the stimulus and the triggering of the bodily response. The later part of the emotion process is a bodily response, for example, changes in heart rate, skin conductance, and facial expression. This description is sufficient to begin an analysis of the emotions, although it does leave out some aspects of the process such as the subjective awareness of the emotion and behaviour that is often part of the emotion response (for example, fighting, running away, hugging another person).
Feeling
Feeling was originally used to describe the physical sensation of touch through either experience or perception. The word is also used to describe other experiences, such as "a feeling of warmth" and of sentience in general (i.e. anything of which one can be aware).
In psychology, the term feeling is closely related to emotion, and usually refers to the conscious subjective experience of emotions.
The neuroscientist Antonio Damasio distinguishes between emotions and feelings: Emotions refer to mental images (i.e. representing either internal or external states of reality) and the bodily changes accompanying them, whereas feelings refer to the perception of bodily changes. In other words, emotions contain a subjective element and a 3rd person observable element, whereas feelings are subjective and private.
Mood
In psychology, a mood is an affective state. In contrast to emotions or feelings, moods are less specific, less intense and less likely to be provoked or instantiated by a particular stimulus or event. Moods are typically described as having either a positive or negative valence. In other words, people usually talk about being in a good mood or a bad mood. There are many different factors that influence mood, and these can lead to positive or negative effects on mood.
Positive mood can be caused by many different aspects of life as well as have certain effects on people as a whole. Good mood is usually considered a state without an identified cause; people cannot pinpoint exactly why they are in a good mood. People seem to experience a positive mood when they have a clean slate, have had a good night sleep, and feel no sense of stress in their life.
Negative moods have been connected with depression, anxiety, aggression, poor self-esteem, physiological stress and decrease in sexual arousal.
Note: The definition of emotion is based on the definition in Internet Encyclopedia of Philosophy. The other definitions are based on articles in Wikipedia.
Affect
Affect, in psychology, refers to the underlying experience of feeling, emotion or mood.
What is emotion?
The video below from Khan Academy tries to answer the question "What is emotion?"
Theories of emotion
Some of the theories of emotion are introduced in the following videos.
Neuroscience of emotion
There following Khan Academy videos give a brief introduction of the neuroscience that relates to emotion.
- Emotions: limbic system: https://youtu.be/GDlDirzOSI8
- Emotions: cerebral hemispheres and prefrontal cortex: https://youtu.be/TQ51Gsb98ec
- Autonomic nervous system (ANS) and physiologic markers of emotion: https://youtu.be/yIWKrQTznXc
