To summarize, according to the W3C, a selector is a chain of one or more simple selectors separated by combinators. Combinators are: whitespace, ">", and "+". Whitespace may appear between a combinator and the simple selectors around it.
The W3C also states that one pseudo-element may be appended to the last simple selector in a chain.
Much of the rest this document will use text called Lorem Ipsum, rather than meaningful information.
Lorem Ipsum is dummy text that has been used by the printing and typesetting industry ever since the 1500s. It has its roots in a piece of classical Latin literature from 45 BC, de Finibus Bonorum et Malorum by Cicero.
There is a Lorem Ipsum generator at http://www.lipsum.com/ that produces realistic-looking text.