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What Is A Selector?
When a style sheet is abstracted from the document tree it applies to (as it is with Embedded and External style sheets), a method is needed to connect style statements to items in the document tree. In the CSS Rule-Set syntax, a 'Selector' is used to specify the portion of the document tree that the style declaration that follows will be attached to.

A Selector is the portion of a Rule-Set that comes before the style Declaration Block embedded within curly braces ('{' and '}'.) Selector syntax in CSS2 can look for matches using five different contexts: elements, attributes, special HTML attributes, Pseudo-Classes and Pseudo-Elements.

The granularity and power that the CSS2 Selector syntax allows gives an author tremendous power, but much of this has not been implemented by the popular browsers. The Selector syntax specified in CSS1 is the norm, but is more limited. Many of the new capabilities in CSS2 help generalize the Selector syntax to make it more flexible with non-specific markup languages like XML. While the expanded syntax is not yet supported, it should not be too long before it is added to CSS-capable browsers.
A Selector is composed of a series of one or more selector components separated by spaces and/or the ">" and "+" selector combining symbols. The components of a selector are described and linked below.
Selector Component = ("*" | [Element Selector])? ([Attribute Selector] | [ID Selector] | [Class Selector] | [Pseudo-Class] | [Pseudo-Element])*
Selector Grouping
CSS1 | CSS2 | IE3 | N4 | O3.5
A Declaration block may be assigned to multiple Selectors by separating the Selectors with commas.
h1 { color: green }
h2 { color: green }
h3 { color: green }
     Becomes =>      h1, h2, h3 { color: green; }
Selector Types
CSS1 | CSS2 | IE3 | N4 | O3.5
Element Selectors
Universal Selector  "*" Matches any element
Simple Element Selector [Elem]Matches specified Element name
Descendent Selectors [Elem1] S+ [Elem2]Elem1 is the direct or indirect ancestor of Elem2
Child Selectors [Elem1] ">" [Elem2]Elem1 is the direct parent of Elem2
Adjacent Selectors [Elem1] "+" [Elem2]Elem1 and Elem2 share the same direct parent AND are next to each other in the document tree
Attribute Selectors
Simple Attribute Selector  [Attr] or
  Matches all elements with the specified attribute or all 'Elem's with the specified attribute
Attribute Value Selector [Attr="value"] or
Matches all elements with the specified attribute/value or all 'Elem's with the specified attribute/value
One-of-many Attribute
Value Selector
[Attr~="value"] or
Allows a portion of an attribute value to be exactly matched where the portions are separated by spaces.
Hyphen Attribute
Value Selector
[Attr|="value"] or
The attribute value is a hyphen-separated list and begins with "value"
Special HTML Selectors
Class Selector  .[Class Name] Matches elements with CLASS attributes carrying a [Class Name] value
ID Selector #[ID Name]Matches the element carrying the unique ID attribute value of [ID Name]
Pseudo-Class Selectors
Link  A:link  Applies to an unvisited hyperlink
Visited A:visitedApplies to a visited hyperlink
First-child [Elem]:first-childApplies to the first child/descendent of an element
Active [Elem]:activeThe state during pointer activation ( eg: press and release of a mouse)
Hover [Elem]:hoverThe state during pointer movement within the rendering region of an element
Focus [Elem]:focusThe state during which an element accepts keyboard input
Lang [Elem]:lang([Code])The selector content uses the specified language
Pseudo-Element Selectors
Before   [Elem]:before  Addresses content that is generated before an element
After [Elem]:afterAddresses content that is generated after an element
First-letter [Elem]:first-letterIndicates the first rendered letter/character for a block-level element
First-line [Elem]:first-lineIndicates the first rendered line on the output device of a block-level element
Notes Browser Peculiarities
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