REST is 
* a software architectural style for simple web-based interfaces that communicate over HTTP and use XML response data. 
* an acronym for REpresentational State Transfer, indicating a stateless client/server protocol in which each request contains all the information necessary to understand the request. 
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An important concept in REST is the existence of sources of specific information, each of which can be referred to using a global URI identifier. In order to manipulate these resources, components of the network (clients and servers) communicate via a standardized HTTP interface and exchange representations of the actual documents conveying the information. 

Any number of clients, servers, caches, tunnels, etc. can mediate the request, but each does so without "seeing past" its own "layering", another constraint of REST and a common principle in many other parts of information and networking architecture. Thus an application can interact with a resource by knowing two things: the identifier of the resource, and the action required. The application does not need to know whether there are caches, proxies, gateways, firewalls, tunnels, or anything else between it and the server actually holding the information. 

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External links:
* http://www.ics.uci.edu/~taylor/documents/2002-REST-TOIT.pdf
* http://en.wikipedia.org/wiki/REST 
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CategoryDefinition