iCloud backs up everything on your iPhone like iTunes would, but doesn't require you to plug it into a computer and it is all available "on the cloud" rather than stuck on your computer. You could use either one, or you could use both, but iCloud is much easier to use and is faster than using iTunes.
you said in your last post
you are never deleting any backup. You are simply not using some of the backup when you restore your phone. It will still be there if you ever need or want it. The things you will not put on your fresh restored iPhone are the things that make your phone slow such as app, safari, and system cache and settings that you've changed. Everything else can be redownloaded and reinstalled including mail, photos, apps, music,etc.
When you decide to restore your iPhone, you would start out by going to Settings > iCloud > Backup and Turning it On. You want to make sure everything that you would like to backup is switched on so that it saves. You can also go onto iTunes on you computer and backup your data there if you want 2 backups just in case. Once it's done backing up just go into iTunes and restore. Everything will be saved to iCloud and iTunes and you won't lose anything. once you get on your freshly restored iPhone, you can't "restore from iTunes/iCloud Backup" you have to "setup as new iPhone." Sign into iTunes and iCloud after that and your iPhone will be reattached to your AppleID. Then it's a matter of only syncing pictures, reinstalling music, and reinstalling apps, mail, etc.. Just make sure you are not restoring your iPhone from a previous backup and you will be good.