What's nginx doing?

Ngix is a Web server, like Apache, it can do dynamic request forwarding, web-side load balancing, reverse proxy, etc.
tomcat is an application server, of course, if not with Shilang, also can do Web server use, it is mainly for the servlet container, Generally used in the application layer, running background logic code, access to the database server, and so on, commonly

used Web sites, the use of apache+tomcat+ database or ngix+tomcat+ database of the three-tier physical structure; if it is a large web site application, there will be a layer of hardware load balancing F5.

from the performance point of view ngix performance seems to be slightly better than Apache, but from the ease of use, Apache configuration is more convenient, powerful, because the Apache seems to be more famous, with more people.