A selector is the bit of a CSS definition that says what the styles are applied to. There are many things that you can do with selectors. The selectors are essentially the same, they appear in the same place in the CSS document, no matter what type of element they are referencing. This would apply the style to all of the IMG HTML elements, images. This is essentially the same method for the other three elements.
Nested selectors are powerful.
Nesting is a concept in HTML where one element is 'nested' within another element. For example bold text is nested within a paragraph, and italic text may be nested within the bold text. You can nest CSS selectors so you can apply a style to an element based on where it appears in the document. To use the selector of an element with a specific class you put the class selector directly after the element selector, with no space. Later on in the document though you can redefine parts of an element.