External objects are similar to custom objects, but external object record data is stored outside your Salesforce organization. External objects are available with Salesforce Connect and Files Connect.

External objects are best used when you have a large amount of data that you can’t or don’t want to store in your Salesforce organization, and you need to use only a small amount of data at any one time.

See “Define External Objects” in the Salesforce Help for how to create and modify external objects.

SOAP API Developer Guide - External Objects