What is the different between ALE, IDOC and BAPI?

ALE

ALE is SAP proprietary technology that enables data communications between two or more SAP R/3 systems and/or R/3 and external systems. When a new enterprise resource planning (ERP) solution such as R/3 is implemented, companies have to interface the ERP system with legacy systems or other ERP systems.

ALE provides intelligent mechanisms where by clients can achieve integration as well as distribution of applications and data.

ALE technology facilitates rapid application prototyping and application interface development, thus reducing implementation time.

The ALE components are inherently integrated with SAP applications and are robust, leading to a highly reliable system.

ALE comes with application distribution/integration scenarios as well as a set of tools, programs, data definitions, and methodologies that you can easily configure to get an interface up and running.

BAPI

BAPIs provide a stable, standardized method for third-party applications and components to integrate into the Business Framework. These interfaces are being specified as part of SAP's initiative with customers, partners and leading standards organizations. Also, SAP has implemented the emerging Object Application Group (OAG) specifications with BAPIs.