A servlet is a Java programming language class used to extend the capabilities of a web server. It allows developers to create dynamic web content by processing requests and generating responses, typically in the form of HTML. Servlets run on the server side and can handle various types of requests, such as those from web browsers. When a user interacts with a web application, the server invokes the appropriate servlet to handle the request. This process is part of the Java EE (Enterprise Edition) platform, which also includes technologies like JSP (JavaServer Pages) and JSF (JavaServer Faces) for building web applications.