IDoc is the data format used to exchange data with SAP systems. ALE is called a suite of tools used to control and monitor the flow of IDocs in and out the SAP systems.
I recommend to read the great SAP Netweaver for Dummies book. The book is based on the curriculum of the basic SAPTEC course of the SAP academy and gives a very good overview how the NetWeaver landscape is built and how it evolved from the celebrated SAP R/3.
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