The Java API is the API for the main Java Library.

The Servlet API is not specifically used for connecting to databases. Instead, servlets typically interact with databases through JDBC (Java Database Connectivity), which is a standard Java API for database access. Servlets can use JDBC to establish connections, execute SQL queries, and retrieve results from a database.

Java Servlet is used for Server Side programming for developing Web Applications. It easily employs Database Connectivity. We can also use JSP however it cannot replace a Java Servlet.

Java Applet is an application designed to transmit on internet to execute on java compatible browsers. Java Servlet is a server side program used to provide services to clients.

Networking is a basic action. A servlet in Java is a single part of networking, a single task.