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Amit Chaudhary 8Amit Chaudhary 8 

Release 15 , A New Relationship Data type (External Object and Data source)

External Lookup Relationship could now be setup between external objects and Salesforce object.
External Object - They are same as custom objects but used to map to data located outside Salesforce.
External object take data from External source and External source obviously has its own DB. 
To create external object : From Setup, click Develop | External Objects.
To create external Data source : Develop | External data source
 
Best Answer chosen by Amit Chaudhary 8
Amit Chaudhary 8Amit Chaudhary 8
External objects are similar to custom objects, except that they map to data located outside Salesforce. Each external object relies on an external data source definition to connect with the data outside Salesforce. Each external object definition maps to a table that contains the data, and the object fields map to accessible table columns. The data in the external table can be searched and referenced in Salesforce using custom tabs and federated search.


1) Each organization can have up to 100 external objects.


External Lookup Relationship could now be setup between external objects and Salesforce object.
External Object - They are same as custom objects but used to map to data located outside Salesforce. External object take data from External source and External source obviously has its own DB. 

To create external object : From Setup, click Develop | External Objects.
To create external Data source : Develop | External data source

All Answers

PratikPratik (Salesforce Developers) 
Hi Amit,

You can find the details:
https://help.salesforce.com/apex/HTViewHelpDoc?id=external_object_define.htm&language=en_US

Thanks,
Pratik
Amit Chaudhary 8Amit Chaudhary 8
External objects are similar to custom objects, except that they map to data located outside Salesforce. Each external object relies on an external data source definition to connect with the data outside Salesforce. Each external object definition maps to a table that contains the data, and the object fields map to accessible table columns. The data in the external table can be searched and referenced in Salesforce using custom tabs and federated search.


1) Each organization can have up to 100 external objects.


External Lookup Relationship could now be setup between external objects and Salesforce object.
External Object - They are same as custom objects but used to map to data located outside Salesforce. External object take data from External source and External source obviously has its own DB. 

To create external object : From Setup, click Develop | External Objects.
To create external Data source : Develop | External data source
This was selected as the best answer