In what context are you asking this (e.g. did you see something in a white paper or something?)
QlikView can certainly integrate data from disparate systems simply by loading in data from different sources. E.g. I can LOAD in a table from a SQL Server database, an Oracle database, and say an Excel spreadsheet all into a single QVW file. Once the data is loaded in QlikView, it doesn't matter from where it was sourced.
I gather this probably isn't what you looking for...please clarify.
Sure thing ~ just to be clear, creating QVD's isn't necessary. Where these ARE valuable is for reusability and in particular speed. QVD's are stored as as a file with an XML header (metadata) and the actual data is in a binary format. Reading QVD's back into QlikView is much more efficient than, for example storing the data in text files. Many folks use QVD's a sort of data warehouse for numerous user-facing QVW files.
Creating QVD files is usually done using a standalone QVW file that reads in the data (e.g. from an ODBC source), using the STORE function, and then drops the table from the QVW (so it isn't wasting any space.) All of this is done in the QlikView script. See the QlikView help for "QVD files" for lots more information.
Reading QVD's into your user-facing apps is not really any different from, say, an Excel file. Just use the "Table Files" wizard in the script editor.
Let me know if that didn't answer you question. -Isaiah