What is "selectors"

A CSS selector is a statement in a format that "matches" all elements that follow that format in the document tree. When all the conditions set in the declaration format are met the "home" selector with the element (or elements) in the document and the rules written in the selector are applied. Consider the very simple CSS rule written below:

    p { color:#f00; }

The selector is the part of the CSS rule that is before the "{" (opening key) sign. The selector here is p, which "matches" all the p elements of the document and makes any text within a paragraph red. Pretty basic.