What is servlet in java? And how to make program in servlet.

Oracle on servlets

basically, Servlets are classes that extends HttpServlet and that can be used (in combination with jsp) to create webapplications.

if you want to learn more about them, I would recommend this book.

if you start creating them, questions might be quicker answered in this forum.

Java Servlet technology provides Web developers with a simple, consistent mechanism for extending the functionality of a Web server and for accessing existing business systems.

you make website with it, its kind of like php. Just search "jsp" in google.

not really. the "website" itself is written in (usually) jsp. the servlets are used for the business logic that's behind it.