A servlet in Java is a server-side component that receives requests and generates responses based on those requests. It is a Java class that is used to extend the functionality of a web server or application server.

When a client makes a request to a web server, the server dispatches the request to the appropriate servlet based on the URL specified in the request. The servlet then generates a response based on the request and sends it back to the client. Servlets are often used to generate dynamic web pages, process form data, handle user authentication, and perform other server-side tasks.

Servlets are part of the Java Servlet API, which provides a set of interfaces and classes for developing web applications. The API includes a standard set of HTTP request and response objects, as well as classes for handling session management, cookies, and other common web application features.

To develop a servlet, you typically create a Java class that extends the javax.servlet.http.HttpServlet class and implements the doGet or doPost method. These methods receive HTTP requests and generate HTTP responses. You then package the servlet class in a Java web application archive (WAR) file and deploy it to a web server or application server.

Overall, servlets are a fundamental technology for developing Java-based web applications, and they provide a powerful and flexible way to generate dynamic content and handle user requests.