What is the work of querySelectorAll in JavaScript?

The querySelectorAll() method in JavaScript returns a Node list containing one or more elements that match a specified CSS selector(s) in the document.

This method is very useful when you want to select a group of elements in the document to manipulate them all together, for example to batch add event listeners or set styles.

The difference between querySelectorAll() and other selection methods like getElementsByClassName or getElementsByTagName is that querySelectorAll() allows you to select elements with much more specificity and flexibility, because it accepts any CSS selector as its parameter.

Here's an example usage:

```javascript
// Select all div elements with class "myClass"
var elements = document.querySelectorAll("div.myClass");

// Now `elements` is a Node list of all matching elements.
// You can iterate through this list and manipulate the elements.
for(var i = 0; i < elements.length; i++) {
elements[i].style.backgroundColor = "red";
}
```

In this example, all <div> elements with class "myClass" will have their background color changed to red.