Virtual DOM library is a JavaScript library that allows developers to create and manipulate the Document Object Model (DOM) of a web page. It enables developers to create dynamic user interfaces without having to directly interact with the DOM. Virtual DOM libraries are used in frontend development for creating interactive user interfaces, such as single-page applications, and for manipulating existing HTML elements on a page. They can be used to add, remove, or modify elements on the page in response to user input or other events.