In January 2009, jQuery's creator John Resig introduced a new open source JavaScript project called Sizzle . A standalone CSS selector engine , Sizzle was written to allow any JavaScript library to use it with little or no modification to its codebase. In fact, jQuery has adopted Sizzle as its own selector engine ever since version 1.3.
Sizzle is the component within jQuery that is responsible for parsing the CSS selector expressions we put into the $() function. It determines which native DOM methods to use as it builds a collection of elements that we can then act on with other jQuery methods. The combination of Sizzle and jQuery's set of traversal methods makes jQuery an extremely powerful tool for finding elements on the page. It boasts a large set of features, an extensible architecture, and a finely-tuned set of internal functions for outstanding power, flexibility, and speed.