What a silly question! Everybody knows what emotions are - or do they? As it turns out, emotion science has a history of arguing about what it is that it deals with. However, the situation is not so bad - over time a consensus is emerging that involves some key aspects of what emotions, as studied by scientific research, are. In my opinion much of the discussion and debate is of historical interest and not of practical significance. However, important emotion researchers still differ with regard to what they feel is part of emotion and what not, what the functions of emotions are, and other aspects (see, Izard, 2010; Widen & Russell, 2010).

In everyday language, emotions are often seen as synonymous for feelings. However, today, most scientists will also look for changes in the activity of the body, including the brain. Some of these bodily changes can be seen, such as a blush, or a smile, and others cannot, such as a change in blood pressure. Often researchers will also emphasize that emotions cause changes in action tendencies, for example getting ready to move away, or explicit actions, such as hugging someone. These different aspects of emotions are also often referred to as components of emotional reactions. Some researchers in the 20th century believed that emotions are packages or programs that will lead to very stereotypical changes, in the face, in other parts of the body, and in how we feel. However, now that much empirical research has been done it appears that the components are only loosely connected. When researchers try to measure emotions, they will assess more than one component.

In scientific terms, emotion is a construct. In using constructs, we give a single name to a bunch of related processes because it facilitates talking about them. Yet, when we look very closely to distinguish one construct from another, it becomes obvious that it is not easy to clearly define the boundaries of such things. This is not unique to emotion, but consider terms such as intelligence, democracy, market, family, or impressionism. Not only does it take more than a paragraph to define such a complex construct, our understanding is in always in flux as new findings appear and scientific exchanges lead to a better understanding of the phenomena we look at.

Emotion, motivation, and cognition are three psychological terms that relate in interesting ways to one another.

References
Izard, C. E. (2010). The many meanings/aspects of emotion: Emotion definitions, functions, activation, and regulation. Emotion Review, 2, 363–370.
Widen, S.C. & Russell, J.A. (2010a). Descriptive and Prescriptive Definitions of Emotion. Emotion Review, 2, 377-378.