Most serious programs need to save data in one form or another and there are lots of ways to accomplish this.
I'm going to divide this data into three types - 1 - Program preferences, 2 - Temporary data created by one part of the program and used by another part, 3 - Data records (ie. relational database)
For number 3 the answer is (relatively) straightforward - use a DBMS (database management system) such SQL Server, MySQL, MS Jet (as used by the Wessex Programs) or similar. All the complex database work is taken care of and there is a wealth of documentation to fall back on. (This may be the subject of a future posting.)
But for type 1 and 2 data, the answer is anything other than easy.
Program preferences traditionally have used the "Registry" to store values.
For those who haven't come across the Registry it is a (big) file where everything from the default Word font to the hardware available is stored. This is a really important file and changes to it could stop your computer working, that said if you (in XP) click "Start" - "Run" and then type "regedit" in the box you will see the registry in all its glory, - don't change anything though!
Let's not get too technical here - what's an XML file? XML stands for Extensible Markup Language and can take the form Key - Sub-Key - Value, eg. "Glass" (key) "Standard Glass" (sub-key), "1.2" (value). Thus making it especially useful for data files.
But by using the XML format all this is avoided - instead of dealing with a preference file "user.xml" a file called "jobs.xml" (then "job1","job2" & so on) is saved until needed by the database and then deleted.