A RESTful resource is anything that is addressable over the web. By addressable, we mean resources that can be accessed and transferred between clients and servers. Subsequently, a resource is a logical, temporal mapping to a concept in the problem domain for which we are implementing a solution.

Here are some examples of REST resources:

• A news story
• The temperature in NY at 4:00 p.m. EST
• A tax return stored in the IRS database
• A list of code revision history in a repository such as SVN or CVS
• A student in a classroom in a school
• A search result for a particular item in a web index, such as Google

Even though a resource's mapping is unique, different requests for a resource can return the same underlying binary representation stored in the server. For example, let's say we have a resource within the context of a publishing system. Then, a request for the latest revision published and the request for revision number 12 will at some point in time return the same representation of the resource. In this example, the last revision is version 12. However, when the latest revision published is increased to version 13, a request to the latest revision will return version 13, and a request for the revision of version 12 will continue returning version 12. This implies that in a RESTful architecture, each resource can be accessed directly and independently, and sometimes, different requests may point to the same resource.

As we are using HTTP to communicate, we can transfer a variety of data types between clients and servers, as long as the data type used is supported by HTTP. For example, if we request a text file from CNN, our browser receives a text file. If we request a Flash movie from YouTube, our browser receives a Flash movie. The data is streamed in both cases over TCP/IP and the browser knows how to interpret the binary streams because of the Content-Type header present in the HTTP response header. Following this principle, in a RESTful system, the representation of a resource in the response body depends on the desired internet media type, which is specified within the request header sent by the client.