Magnolia's REST Web service allows you to manipulate content through a Web API. You can create, read, update and delete nodes in the JCR. The nodes can be pages, components, contacts or anything else that is stored in a named workspace. You can also execute commands to activate, export and import content.
The REST service is similar to the Data module in Magnolia 4.5 which pulled data from other sources into Magnolia. However, REST works with the push principle where your application communicates with a Magnolia REST endpoint and exchanges a representation of a resource such as an XML of a page. REST is useful for connection tasks. Use REST to push product data from a third party system into Magnolia and let editors enrich it in a Magnolia app, for example.