Tomcat began at Sun Microsystems as the reference implementation of the Java Servlet and JSP specifications, written by James Duncan Davidson. Sun donated it to the Apache Software Foundation in 1999, and it became the default way to run Java web applications outside a full application server.
Each major Tomcat version implements one generation of the Java EE, now Jakarta EE, specifications. The biggest change came with Tomcat 10, which moved from javax.* to jakarta.* package names, so older applications need migrating. Several major versions are maintained in parallel, with patch releases about every month.