There are many answers to the question what it is: Belief. One valid definition of what belief is, is this one:
Belief is finding truth in something without direct evidence, accepting something for fact when its truth cannot be or has not been proved.
From belief we can go a step further into faith. Faith is more than believing something to be true, it is entrusting and committing oneself to it.
Belief and faith are important factors in human life. For instance, commercial transactions cannot take place unless people trust each other, which is not always based on facts and past experience but also on intuitive gut feeling. People trust their own self image when they choose a carreer, and their feelings when choosing a partner for life. There is another path that gives belief, and especially faith, a more complex meaning, and that is the religious dimension. For everybody it is possible to understand faith in other people, in the self, even in nature. It is relatively easy to trust the visible, empirical world that we know. However, not everybody is willing to find truth in an unseen godly power who created everything we know: A hereafter, an evil force, a (or the) future, or a divine judgement.
Belief indeed is one side of the coin and knowledge is the other. We can see these concepts as opposites but also as twin sisters. What is knowledge?
Knowledge is the possession of facts that people have, based on open observation that can be shared by others, or on logical deduction.
Belief stops as soon as we have knowledge.
We may call observations evidence to a phenomenon. And evidence is information that we can share with other people in an unambiguous way. They observe exactly the same phenomenon as we do. A picture taken of a football match gives information to other people about its proceedings. A secretary takes notes during a meeting, and those present agree afterwards with their contents, in order to record the talks. When the street is wet, we can conclude that either rain has fallen, or the cleansing-service passed by. Some observations are household, such as the fact that water boils at a temperature of a hundred degree Celsius. Others are more sophisticated, like the neutron structure of gold, and such knowledge is agreed to be called science. After all, not everybody is trained to interpret sophisticated observations. However, the easy part of knowledge is that everybody can endorse its truth, once more people have observed the occurrence of the facts, and once people can re-observe the facts anytime. Knowledge is the answer to the how to-questions that people, curious as they are, have posed themselves through the ages. The fact that our knowledge is still incomplete, gives us the impetus to search further. To a certain extend, it is possible to answer the how to-questions. Our technical skills have grown over the generations, and we are able to reach further and deeper. Now we can observe the very small or very far phenomena. We can accept that there are certain limits to our ability to answer the how to-questions and blame our inabilities to our lack of knowledge. We hope that a new scientist may find new answers. We don't know the limit of our abilities, nor if our knowledge gathering may be confined by unknown barriers that have nothing to do with our capabilities. A good example is the Biblical Babel Tower built by the people in order to see God. God set boundaries to the gathering of human knowledge: the people were not allowed to see Him, so He destroyed the tower and from then the people spoke different languages. Apart from the unknown limit to our scientific knowledge, we have the issue of why-questions. As soon as we reach the question of how the universe was created or originated, we meet a problem that has led to various ideological currents, with sometimes long histories and conflicts among each other. Basically it is the history of controversy between science and religion and between competing religions.
Science has tried to answer the how to-questions more or less successfully. More importantly, religions try to answer the why-questions of life.
Why did Mary Blake die so young?
Why did this happen to her?
Why is life such unfair shit?
Why do things never go right?
Why are we on earth?
Why is nature of such unsurpassed beauty and variation?
Why have I become a compassionate conservative Republican?
Why came the dinosaurs first and then the primates?
All these questions are limited by our capabilities and, according to believers, our permissions. Biblical history tells how God sets boundaries to human knowledge. God destroyed Babel Tower, because He did not want man to reach Him. Man also has no complete sight on the world and universe around him. The Islamic Scripture, the Qur'an, too, has verses on how man may and must gather knowledge, however, to the extend that Allah allows him:
"O ye assembly of Jinn and men! If it be ye can pass beyond the zones of heavens and the earth, pass ye! Not without authority shall ye be able to pass!" Q:55:33
"And He has created (other) things of which ye have no knowledge." Q:16:8
"Say: 'Are those equal, those who know and those who do not know?' It is those who are endued with understanding that receive admonition." Q:39:9
Saying that the evolution determines why the dinosaurs precede the primates, is no answer to the why-question. Why-questions give a reason and arise from an approach of subjectivity. They come from human perception and have no scientific foundation. Science cannot answer why-questions, because they touch the areas of choice and preference. Someone or something determined that it was better, or maybe nicer, for the dinosaurs to precede the primates. Science does not determine qualities such as beauty, good taste, morality, better, higher, or worse. Even the occurrence of coincidence does not cut it, because always someone will come and ask: Why did dinosaurs happen to arrive before primates, why did chance choose the dinosaur to precede the primate?
Belief is related to the human mind: The set of thought, emotion, consciousness, ambition, and spirit, qualities that scientists have been unable to determine as yet. What are the spirit and our feelings, what is our personality, our consciousness, where do they reside, what are their form and look? One thing that we can say, is that thoughts, emotions, ambition, awareness, and belief are components and instruments of a relationship. A relationship with ourselves, other people, animals and plants, and with metaphysics or abstract concepts. We may define our soul and consciousness as our invisible self and our personality as its link with the outside world. Thinking, emotions, ambitions, and beliefs link us to the self or the other on an invisible level. Many artists make it their job to give thought, emotion, belief, spirit, and personality a concrete expression in image, text, or sound. But also the artist stumbles on the invisibility and elusiveness of these topics. They are the aspects of life that perhaps matter the most, yet they cannot be reproduced and are therefore no part of scientific knowledge. Their behavioral consequences, however, are often very visible and tangible and follow a repeatedly shared pattern, which we may call a structure. This is the reason why relations with the self and other people are open to the scientific approach, to a certain extend. This also goes for our relations with other living creatures, our environment, and with materials. When people have a problem with themselves, psychologists and psychiatrists jump into the field to assist them. When people have problems with others, not only psychologists but also the legal system has to assist, sometimes against the person's will. We know relatively much about how the human mind works and can make valid statements about many phenomena.
However should we not forget, that many things involving the human mind are unknown to us. This also has implications for religious issues: Our connection with metaphysics. Can a divine power communicate with people, how does this work, can we talk with an invisible unknown power? Is it therefore wise to discard any divine revelation to prophets as fraud? We have no scientific answer to that.