Selector are the basic component in the CSS. Here's a definition of the selector from W3C documentation -
A Selector represents a structure. This structure can be used as a condition (e.g. in a CSS rule) that determines which elements a selector matches in the document tree, or as a flat description of the HTML or XML fragment corresponding to that structure. - W3C
Below is some example of some rich contextual representations of CSS3 selectors.