Web server vs application server
Web server
A Web server can process HTTP requests and respond with HTML pages, provided the requests are handled by server side programs
It uses HTTP protocol to transmit the request/responses. Here Web server handled both request/response and business logic. This is more rigid design.
A web transaction using web server will be like this as below,
Browser (click operation to display some results on a grid> ==> Request goes to web server (Apache HTTP server) ==> Database
Application server
An Application server controls the business logic. Separating business logic from web server and adding to application server will give more reusability for this logic across different applications. This is more flexible design.
A web transaction using app server will be like this as below,
Browser (click operation to display some results on a grid> ==> Request goes to web server (Apache HTTP server) ==> Application server (Tomcat server) ==> Database
Or
Browser (click operation to display some results on a grid> ==> Request goes to web server (Apache HTTP server) ==> Application server (Tomcat server) ==> Web service ==> Third party Database
Please click on below linking for getting quick summary
Web server vs application server vs database server