A selector is used in jQuery to select DOM elements from a DOM document. That document is, in most cases, the DOM document present in all browsers, but can also be an XML document received via Ajax.
The selectors are a composition of CSS and custom additions. All selectors available in jQuery are documented on the Selectors API page.
There are lot of plugins that leverage jQuery’s selectors in other ways. The validation plugin accepts a selector to specify a dependency, whether an input is required or not: