How can you know which file to backup and which one not to backup?

"If you're referring to information that you originally saved to a website or web server, contact the webmaster or server's host for help retrieving your files from their database. If you're...

you copy all the original files to a another hard disk or flash drive before formatting the disk with the original files

Just another copy of the file.

The files which store the copy of your original data are called backup files. In windows backup files are created by NTbackup utility and windows backup and restore utilities. These backup files have...

internal backup is when you back up your files to your hard drive not an other device! to back up you need to download some software fron the internet