As well as the files that make up a database, there are various other files that are associated with your database, or DP4 more generally. These are:
Backup files are (or should be!) created periodically, and are normally stored on tape or other backup media.
DP4 creates various work files when you run a program
Spooler files are created for some operating systems for background printing
This log records all uses of the DP4 programs, including user name, the time and date, the name of the programs run, and any DP4 errors
Extract files are temporary files created during report printing
Print files are created when you select the option to send printer output to a file. They contain the text of the report or form that has been printed