<p>I've seen news of <a href="http://github.com/jeresig/sizzle/tree/master" rel="noreferrer">John Resig's fast new selector engine named Sizzle</a> pop up in quite a few places, but I don't know what a selector engine is, nor have any of the articles given an explanation of what it is.  I know Resig is the creator of jQuery, and that Sizzle is something in Javascript, but beyond that I don't know what it is. So, what is a selector engine?</p>

<p>Thanks!</p>

A selector engine is used to query a page's DOM for particular elements, based on some sort of query (usually CSS syntax or similar).
For example, this jQuery:
```
```
$('div')
```
```
Would search for and return all of the <div> elements on the page.  It uses jQuery's selector engine to do that.
Optimizing the selector engine is a big deal because almost every operation you perform with these frameworks is based on some sort of DOM query.