That depends on the storage format use by the backup application in question. On Unix/Linux there are a few formats that could qualify as de-facto standards, like compressed tarfiles and cpio archives. On the Windows platform, things are not as straightforward:
- The backup application in Windows 2000/XP/2003 (known as "NTBackup") uses a proprietary format, identified by the .bkf file extension when backing up to a file. Tapes and file-based backups are supported.
- Backup Exec (currently owned by Symantec) is probably the most popular backup application on the Windows platform in the SMB market segment. It uses a proprietary backup format, but can read backups created by NTBackup (as NTBackup is an earlier, licensed version of Backup Exec). In addition to tapes and files, various virtual tape libraries are supported (requires additional licenses).
- Windows Backup in Windows 2008 and later is a completely different application. It backs up entire volumes to what is basically an image file. It can only store backups on dedicated disk drives (support for network shares was added in Windows 2008 R2).
- There are quite a few other backup applications on the market, and each usually has its own, proprietary backup/file format (generally true for the commercial applications).
Of course, you don't have to use a particular backup application to migrate files from a Windows PC/server to a Linux system. You could simply create a zip archive and store it on a removable drive. You could also copy the files across the network with rsync, samba, ftp, or any tool suitable for the job.