MooTools' selection awesomeness is further awesome'ized in its ability to utilize pseudo-classes. Pseudo-class selectors are special selectors that allow you to find items from the DOM using properties that can't be easily deduced using ordinary ID, class, tag, or attribute selectors. Psuedo-class selection will allow you to write complex selection strings; ones that would involve a ton of code if you were to write them in native JavaScript syntax.
A pseudo-class, in CSS3 specifications, allows you to select DOM elements that are difficult to select using normal selectors. The pseudo-class concept was introduced as part of CSS3 specifications by the W3C organization (the people in charge of establishing web standards).
Using a pseudo-class is simple, and follows the format:
:pseudo-classname
Or
:(pseudo-classname)