Describe the JSP custom tag library.

JSP custom tag library allows you to define your own custom tags that can be used within JSP pages. Custom tags encapsulate reusable functionality and can be designed to perform specific tasks or provide custom behavior. They are defined in a tag library descriptor (TLD) file, which specifies the tags, their attributes, and the corresponding tag handler classes. Custom tags offer a way to extend the functionality of JSP and promote code reuse and modularity.