A servlet is a Java programming language class that is used to extend the capabilities of a web server. It allows developers to create dynamic web content by processing requests from clients, typically web browsers, and generating responses. Servlets run on a server and can handle complex tasks like interacting with databases or managing user sessions. When a client sends a request, the web server forwards it to the appropriate servlet. The servlet processes the request, often using JavaServer Pages (JSP) or other technologies, and then sends back an HTML response. This makes servlets a key component in building web applications in the Java EE environment.