Similar to the way applets run on a browser and extend the browser's capabilities, servlets run on a Java-enabled Web server and extend the Web server's capabilities.
For example, servlets can support dynamic Web page content, provide database access, serve multiple clients at one time, and filter data by MIME type.
Servlets are Java programs that use the Java Servlet Application Programming Interface (API). They can exist as separate class files (compiled java files), or can be included in a JAR (Java ARchive) file or WAR (Web ARchive) file.
For the purposes of IBM WebSphere Application Server, discussions of servlets focus on HTTP servlets, which serve Web-based clients. Non-HTTP servlets are possible, but are not central to solutions providing Web-based e-business transactions.