In the world of computer networking, a server is a program which operates as a socket listener. The term server can also be used generally to describe a host which has been deployed to execute one or more such programs. Hence, in theory, any computerized process which shares a resource with one or more client processes is called a server.
If we have to talk about a server in terms of hardware, then a server means a computer that is intended to run software applications under heavy demand of a network environment.
If we have to look at the client-server configuration, there are one or more machines, either a computer or a computer appliance, which share information with each other, where one acts as a host for the other.
What is an Application Server?
An application server is a software framework, which is dedicated to the efficient execution of programs, routines, or scripts to support the construction of applications. This term was originally used to describe client-server applications and servers, which ran SQL services and middleware servers, so as to be able to differentiate them from file servers.
It was later that the term came to be used in the context of web applications. However, it has evolved into a more comprehensive service layer. An application server is nothing but a set of components, which are accessible to the software developer through an API defined by the platform itself.
In web applications, these components are often in the same machine as the web server as its main job is to support the construction of dynamic pages. However, now they do not just target the generation of web pages, they also implement services like clustering, fail-over, and load balancing. Due to this, developers can focus on implementing business logic.
What is a Web Server?
A web server is a computer program which delivers content, like web pages, using the Hyper Text Transfer Protocol (HTTP), over the World Wide Web. The term may also stand for a computer or a virtual machine, which runs a program.
The communication is initiated by the client, commonly called a web browser or web crawler. By making request for a specific bit of information using HTTP, the request is met with a response by the server and the content is sent over.
In case of a problem, an error message is sent back. The server also receives content from clients. Such a feature is often used for submitting web forms and for uploading files.
Application Server Vs. Web Server
Like there is confusion about application software and system software, there is also confusion about web servers and application servers. The basic difference between these is that the web server serves pages for viewing in a web browser, whereas an application server gives the required methods that can be called by client applications.
In other words, HTTP requests are handled by web servers and business logic is served to application programs through a number of protocols by the application server.
In an application server, a client can use GUI's and web servers, whereas in web servers the client can use HTML or HTTP. If any functionality is to be added, an application adds the same, however, the same is not the case with web servers.
Web servers do not support or provide the concept of multi-threading. However, in application servers, there are features like connection pooling, isolation pooling, multi-threading, etc.
The most common examples of application servers are the Java Application Server, Weblogic Server, Apache Geronimo, etc. On the other hand, the examples of web servers are Microsoft IIS, Apache, etc.