Specific to the Web, a Web server is the computer program (housed in a computer) that serves requested HTML pages or files. A Web client is the requesting program associated with the user. The Web browser in your computer is a client that requests HTML files from Web servers.

Proxy server

In an enterprise that uses the Internet, a proxy server is a server that acts as an intermediary between a workstation user and the Internet so that the enterprise can ensure security, administrative control, and caching service. A proxy server is associated with or part of a gateway server that separates the enterprise network from the outside network and a firewall server that protects the enterprise network from outside intrusion.

A proxy server receives a request for an Internet service (such as a Web page request) from a user. If it passes filtering requirements, the proxy server, assuming it is also a cache server, looks in its local cache of previously downloaded Web pages. If it finds the page, it returns it to the user without needing to forward the request to the Internet. If the page is not in the cache, the proxy server, acting as a client on behalf of the user, uses one of its own IP addresses to request the page from the server out on the Internet. When the page is returned, the proxy server relates it to the original request and forwards it on to the user.

To the user, the proxy server is invisible; all Internet requests and returned responses appear to be directly with the addressed Internet server. (The proxy is not quite invisible; its IP address has to be specified as a configuration option to the browser or other protocol program.)

An advantage of a proxy server is that its cache can serve all users. If one or more Internet sites are frequently requested, these are likely to be in the proxy's cache, which will improve user response time. In fact, there are special servers called cache servers. A proxy can also do logging.

The functions of proxy, firewall, and caching can be in separate server programs or combined in a single package. Different server programs can be in different computers. For example, a proxy server may in the same machine with a firewall server or it may be on a separate server and forward requests through the firewall.

Cache server

A cache server (sometimes called a cache engine) is a server relatively close to Internet users and typically within a business enterprise that saves (caches) Web pages and possibly FTP and other files that all server users have requested so that successive requests for these pages or files can be satisfied by the cache server rather than requiring the user of the Internet. A cache server not only serves its users by getting information more quickly but also reduces Internet traffic.

A cache server is almost always also a proxy server, which is a server that "represents" users by intercepting their Internet requests and managing them for users. Typically, this is because enterprise resources are being protected by a firewall server that allows outgoing requests to go out but needs to screen all incoming traffic. A proxy server helps match incoming messages with outgoing requests and is in a position to also cache the files that are received for later recall by any user. To the user, the proxy and cache servers are invisible; all Internet requests and returned responses appear to be coming from the addressed place on the Internet. (The proxy is not quite invisible; its IP address has to be specified as a configuration option to the browser or other protocol program.)

Video server

A storage system that provides audio and video storage for a network of clients. While there are some analog systems based on optical disks, most used in professional and broadcast applications are based on digital disk storage.

Aside from those used for video on demand (VOD), video servers are applied in three areas of television operation: transmission, post production and news. Compared to general purpose file servers, video severs must handle far more data, files are larger and must be continuously delivered.

There is no general specification for video servers and so the performance between models varies greatly according to storage capacity, number of channels, compression ratio and degree of access to store material--the latter having a profound influence.

Store sizes are very large, typically up to 500 Gigabytes or more. Operation depends entirely on connected devices, edit suites, automation systems, secondary servers, etc., so the effectiveness of the necessary remote control and video networking is vital to success.

Application server

An application server is a server program in a computer in a distributed network that provides the business logic for an application program. The application server is frequently viewed as part of a three-tier application, consisting of a graphical user interface (GUI) server, an application (business logic) server, and a database and transaction server. More descriptively, it can be viewed as dividing an application into:

• A first-tier, front-end, Web browser-based graphical user interface, usually at a personal computer or workstation
• A middle-tier business logic application or set of applications, possibly on a local area network or intranet server
• A third-tier, back-end, database and transaction server, sometimes on a mainframe or large server.

Reference: Digital Dictionary
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