IDOC is simply a data container used to exchange information between any two processes that can understand the syntax and semantics of the data.
In other words ,an IDOC is like a data file with a specified format which is exchanged between 2 systems which know how to interpret that data.
IDOC stands for " Intermediate Document"
When we execute an outbound ALE or EDI Process, an IDOC is created.
In the SAP System, IDOCs are stored in database.Every IDOC has an unique number(within a client)