To understand how Apache Tomcat is different from Apache you simply need to understand this simple concept: Apache Tomcat is a Java servlet container which works as a webpage server. Remember, a Java servlet is a particular type of Java program that can extend the capabilities of a specific server.
Java servlets can respond to any type of request but Java servlets are generally used to enable applications which are hosted on a web server. These web servlets are Java’s rendition of web content tech that renders dynamic pages, think ASP.NET and PHP for example.
The code for Apache Tomcat was donated by the creators of Java, Sun Microsystems. The Apache Software Foundation received the donation from Sun in 1999 and made Apache Tomcat one if it’s top-level projects in 2005. Apache Tomcat is used to power about 1% of the world’s websites.
There are other open source alternatives if you want a Java application server, including Glassfish, JBoss and Wildfly – these are often listed alongside Apache Tomcat.