how does the classification take place?
It can act on any heritable phenotypic trait, and selective pressure can be produced by any aspect of the environment.
Natural selection
Natural selection can act on any heritable phenotypic trait, and selective pressure can be produced by any aspect of the environment, including sexual selection and competition with members of the same or other species. 
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". 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.
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Natural selection [SEP] Classification
Natural selection can act on any heritable phenotypic trait, and selective pressure can be produced by any aspect of the environment, including sexual selection and competition with members of the same or other species. However, this does not imply that natural selection is always directional and results in adaptive evolution; natural selection often results in the maintenance of the status quo by eliminating less fit variants. Selection can be classified in several different ways, such as by its effect on a trait, on genetic diversity, by the life cycle stage where it acts, by the unit of selection, or by the resource being competed for.