It is an standard intermediate document used in EDI/ALE 
process used in transferring the business objects to 
business partners and in distribution of master data to the 
diff plants which belongs to diff company codes. 

IDoc is nothing but a container of data.
It holds the data which is to be transferred to other 
system.
IDoc holds the data in CHAR format.
just like a word document which is used to send data to 
others, Idoc is used to send Business related data to send 
data to other system.

Idoc is an intermediate document or data container for ALE
and EDI.

      Idoc structure has control records,data records and
status records.

Control record : has the details of partner profil,port etc.

Data records : it has segments which holds the actual
content of the transaction or document.

Status records :it indicates with number in which state your
idoc is.i,e whether successfully created or it has any errors.