About
A resource server is one of the 4 roles of Oauth. It hosts the protected resources, capable of accepting and responding to protected resource requests using access tokens.
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Example
An end-user (resource owner) can grant a printing service (client) access to her protected photos (the protected resources) stored at a photo-sharing service (the resource server).
Implementation
The interaction between the authorization server and resource server is beyond the scope of the oauth specification.