A Web server serves pages for viewing in a Web browser, 
while an application server provides methods that client 
applications can call.
A Web server exclusively handles HTTP requests, whereas an 
application server serves business logic to application 
programs through any number of protocols.

The Web server
A Web server handles the HTTP protocol. When the Web server 
receives an HTTP request, it responds with an HTTP 
response, such as sending back an HTML page. To process a 
request, a Web server may respond with a static HTML page 
or image, or delegate the dynamic response generation to 
some other program such as CGI scripts, JSPs (JavaServer 
Pages), servlets, ASPs (Active Server Pages), server-side 
JavaScripts, or some other server-side technology. Whatever 
their purpose, such server-side programs generate a 
response, most often in HTML, for viewing in a Web browser.
The Web server doesn't provide any functionality beyond 
simply providing an environment in which the server-side 
program can execute and pass back the generated responses. 
The server-side program usually provides for itself such 
functions as transaction processing, database connectivity, 
and messaging.
While a Web server may not itself support transactions or 
database connection pooling, it may employ various 
strategies for fault tolerance and scalability such as load 
balancing, caching, and clustering.

The application server
an application server exposes business logic to client 
applications through various protocols, possibly including 
HTTP. While a Web server mainly deals with sending HTML for 
display in a Web browser, an application server provides 
access to business logic for use by client application 
programs. The application program can use this logic just 
as it would call a method on an object.
The information traveling back and forth between an 
application server and its client is not restricted to 
simple display markup. Instead, the information is program 
logic. Since the logic takes the form of data and method 
calls and not static HTML, the client can employ the 
exposed business logic however it wants.
the application server manages its own resources. Such gate-
keeping duties include security, transaction processing, 
resource pooling, and messaging. Like a Web server, an 
application server may also employ various scalability and 
fault-tolerance techniques.
Additionally, most application servers also contain a Web 
server, meaning you can consider a Web server a subset of 
an application server. While application servers contain 
Web server functionality, developers rarely deploy 
application servers in that capacity. Instead, when needed, 
they often deploy standalone Web servers in tandem with 
application servers.