These files may come as part of the operating system, a third-party device driver or other sources.
Microsoft Windows and MS-DOS mark their more valuable system files with a "system" attribute to protect them against accidental deletion.
Specific example of system files include the files with .sys filename extension in MS-DOS.
In Windows NT family, the system files are mainly under the folder C:\Windows\System32.
In Mac OS they are in the System suitcase.