OData - Open Data Protocol is method to access the information exposed by different data source (Cloud storage, SharePoint, SQL Database, custom application etc) by different client (Web browser, Mobile, Business Intelligence, Custom application- .net/Java etc)
It is abstract data model and protocol used to access the information exposed by any data source. In real time scenario, client application may need to read the data from different source using different data provider. But it is always good to have single point of contact and mode to access the different sources; this can be easily achieved by using OData.