REST stands for "Representational State Transfer". It is just a software architectural style, or a set of design rules that can be used to write web services. Because it is a style, there are no standards defined for this.

How does REST works?
Representational State Transafer means the state of the client changes based on the representation of the resource. Imagine surfing a web page using a URL. When you access the URL, a page (resource) is returned to you. When you select another page, a different representation of the resource is retuned. Based on the the representation returned, the state of the client (calling application) also changes.

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