That depends on your definition of a web server. Tomcat can serve HTML pages and handle CGI but it's capabilities are limited compared to a fully loaded web server such as Apache that was designed to be a web server from the outset.

Tomcat ships with a number of connectors of which one is an HTTP connector. While the Native Tomcat HTTP connector is great, it is limited in functionality compared to a full webserver like Apache. The documentation explains the capabilities and limitations. The documentation also describes why you would choose to implement Tomcat and Apache.

A production Java application environment typically consists of a Java Application Server (e.g. Tomcat) paired with a full web server. This pairing leverages the strengths of both applications.