I have to say that my ideas are pretty much messy and there is so much more to discuss about this topic. SO feel free to comment and give your remarks, as  they will be the basis for the Vietnamese rewritten version of this. Thank you.

What we truly mean by emotions ? It's about feelings, evaluations or motivations ? If it's comprised by all aforementioned, what aspect of its can be controlled ?

Emotions, can be depicted as the process following. When we receive particular influences from the outside, we first conjure evaluations towards the things we receive. They're maybe made so subtle that we hardly notice. But then these judgments lead to certain feelings, like anger, joy, or contempt...These are triggered by related physiological changes in our body, chemical reactions in particular. And not only does the process take action internally but be expressed in the form of facial expressions, as well as certain body reactions. And as feelings are created to help us adapt to our circumstances, we then take actions accordingly, which explains why emotions can be perceived as the basis of motivation.
Regarding how to monitor the emotions, we first date back to the time of John B.Watson - a famous for his theory of classical behaviorism.
Blog Archives - PSYCHOLOGY @ NS

John B.Watson went down in history notoriously for his "Little Albert" experiment. For further information about how it was conducted, take a look at this article : Little Albert. Now, with little regards to other details as the origin of the boy, or what happened afterwards, the conclusion obtained was that not only can human behavior  be predicted—given certain stimuli  and conditions—it can also be  controlled and modified. By what means ? As in this experiment, when the boy was exposed to the white rat at first, he had shown no signs of fear or tremor, also, when Mr Watson struck the metal bar creating a deafening sound near him, the boy became frightened and distressed.  Now that Mr Watson came up with a hypothesis, in which he struck the bar when the boy touched the white rat, in order to cause the fear of the rat in the baby. It proved successful, given that the boy screamed at the sight of the rat, or even something else with the white color.
Then, it can be inferred that feelings are not unchangeable, and that certain circumstances can bring up certain responses depending on what is associated with those. This theory was studied and further developed by Joseph Wolpe
Inheriting the findings of John B.Watson, Mr. Wolpe then applied them to the field of psychotherapy. He came up with the idea that a human being is not capable of  experiencing two contradictory  states of emotion at the same time, that you can't feel fear when you are in a state of relaxation. He went on to put this theory in use by introducing the systematic desensitization technique, by which one may learn to unlearn the feelings unwanted.
Briefly, there are 3 main steps in his system of desensitization. First, the patient and the therapist work together to create a list of anxiety-induced stimuli. Next, they continue to figure out the compatible coping mechanisms. Finally, the therapist links the stimulus to the relaxation method, and step by step helps the patient at ease at the sight of their displeasure.

Back to the time when this method was first invented, Freudian psychoanalysis, which sees internal conflicts as the main source of stress, was still dominant, and  Joseph Wolpe had treated soldiers for anxiety brought on by post-traumatic stress disorder from the experiences ofWorld War II, using psychoanalysis. Talking to these men, Joseph realized that a lengthy introspective analysis of stressful reminiscenes did not stop their pain induction, nor make their memories evanescent. Then, as aforementioned, he applied the classical conditioning of John B.Watson, and succeeded in helping his patients controlling their emotional states.

This method has been widely applied to treat phobias, and still one effective way of facing the anxiety.  As life goes on, we are endlessly exposed to distresses caused by the unwanted and uncontrollable experiences. And it is pleasant to know that you can apply desensitization to heal your wounds.