Sync means to synchronize your iTunes media files on the iPad with the libraries in iTunes on the computer: Music, movies, books, apps, etc. You control what gets synced to the iPad with the settings for each category. You can force iTunes media on the iPad to sync back to the computer using the Transfer item under the Files > Devices menu.
Backup is a separate action, that backs up your settings, camera roll, and the contents of various other apps. It does not backup your iTuns media or the apps themselves. What gets backed up by iTunes is entirely up to Apple and the app developer.
iTunes may or may not synchronize and do a backup automatically when you plug the iPad in. Again, it depends on your settings. You can do just a sync, a backup and sync, or neither.
The manual sync/apply (button at the bottom of the iTunes screen when iPad is plugged in) only syncs the iPad. It does not do a backup.
By the way, your written English has improved a lot since you first posted here. Congratulations. I'm envious. I've never managed to get past a few simple phrases when I attempt to learn a new language.