Emotions permit navigating an organism's complex physical and social environment by allowing to tag objects, actions, and events with meaning and implications for the organism. By linking to memories of the past, analyses of the present, and projections into the future, time is as much a factor as is space in optimizing such navigation. Emotions do that not only by providing information to the individual in the shape of subjective experience, but also by affecting every aspect of processing information regarding external and internal events. Thus, what to pay attention to, or what to remember (or not) is just as influenced by this system as is what elements to associate with other elements in the future. Bodily changes are often involved that might be helpful in supporting certain modes of response. These involve basically all other organismic systems ranging from muscles involved in locomotion, to cardiovascular changes, to modulating digestion and immune function. In this sense, emotions are active because they do not just facilitate dealing with situations, they actively influence them by biasing the own behavior as well as that of other organisms. Expressive behavior affects not only conspecifics, but can also have an effect on members of other species. In humans, a very social species, such expressive behavior leads to coordinated actions in many ways. Meaning can be assigned by direct experience, such as pain or pleasure, by observation of others' emotional responses, as well as by stories of real and fictional events. This allows rapid and slow mediated spreading of emotionally relevant information going beyond the effect of learning by first-hand experience. Thus, meaning is not only constructed by the individual, but also by aggregates of individuals, such as families, clans, companies, religious groups, nations, etc. It is particularly this process that distinguishes human emotions from emotional processes in non-human animals.
Events that are perceived by the organism as being of high relevance can lead to rapid and considerable change in basically every system of the body. Typically, such activation is self-regulating as the type of behavior that is being facilitated by the bodily changes will lead to the self-termination of the emotion-cascade. For example, running away from a threat, scaring away an annoying person, or obtaining comfort. With experience and learning the strategic manipulation of emotion relevant responses allows modulation of the own or others' emotional state, for example using expressive or other behaviors.
OK, then there is the part with different emotions, learning about emotions, what happens when emotions get out of hand, and what happens to emotions when some of the systems that are usually involved do not work well, but that was already a massive chunk of conjecture, so what do you think?