The IDoc interface exchanges business data with an external system.
The IDoc interface consists of the definition of a data structure, along with processing logic for this data structure.
The data structure is the IDoc. The IDoc is the exchange format common to all the communicating systems. You can specify exception handling in the SAP Business Workflow, with IDocs, without the data already having to exist as SAP application documents.
You need the IDoc interface in the following scenarios:
Electronic data exchange (EDI)
Connect other business application systems (such as PC applications, external workflow tools) by IDoc
Application Link Enabling (ALE)
Application Link Enabling (ALE) is a technology to create and run distributed applications.