Web server serves pages for viewing in a web browser while Apps server provides methods that client application can call. The web server exclusively handles HTTP requests where as apps server serves business logic to application program through any number of protocols. Web server cannot handle huge load (handles request/response up to 10,000 request/responses).An apps server is for all types of protocols such as HTTP, FTP, and SMTP so on. An apps server can handle huge request/response.
Application server exposes business logic to client application through various protocols. An application server provides access to business logic for use by client application program. The application program can use this logic just as it would call a method on a object. In most cases the server exposes this business logic through component API such as the EJB component model. Moreover the application server manages its own resources such as security, transactions, resource pooling and messaging.