CSS Selectors

http://www.w3.org/TR/css3-selectors/#selectors

Universal selector => *
ID selector => #myid
Class selector => .myclass
Type (element) selector => p
Attribute selector => E[foo^="bar"]Structural pseudo-classes => E:nth-child(n)
User action pseudo-classes => E:hover
Element state pseudo-classes => E:enabled
Negation pseudo-class => E:not(selector)
Pseudo-elements => E::before

Combinators => Descendant (space), Child (>), Adjacent sibling (+) and General sibling (~)

For HTML, case insensitive.

I also have a post about CSS Performance. Selector efficiency is key to performance in jQuery programming, which basically is to select something, take some actions against DOM.