what can you tell me about its terminology?
The term is most often defined to operate on heritable traits, because these directly participate in evolution.
Natural selection
The term "natural selection" is most often defined to operate on heritable traits, because these directly participate in evolution. 
how does natural selection work as a mechanism of evolution A prerequisite for natural selection to result in adaptive evolution, novel traits and speciation is the presence of heritable genetic variation that results in fitness differences. what is the role of darwin in it? He defined it as the "principle by which each slight variation [of a trait], if useful, is preserved".
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Natural selection [SEP] Terminology
The term "natural selection" is most often defined to operate on heritable traits, because these directly participate in evolution. However, natural selection is "blind" in the sense that changes in phenotype can give a reproductive advantage regardless of whether or not the trait is heritable. Following Darwin's primary usage, the term is used to refer both to the evolutionary consequence of blind selection and to its mechanisms. It is sometimes helpful to explicitly distinguish between selection's mechanisms and its effects; when this distinction is important, scientists define "(phenotypic) natural selection" specifically as "those mechanisms that contribute to the selection of individuals that reproduce", without regard to whether the basis of the selection is heritable.