What is 'Not Enough Storage' supposed to mean?
Your iPhone and iPad are clever devices equipped with mechanisms to keep all your precious content safe, even if you don't use a computer or your phone is stolen. This is one of the advantages of iCloud, a brand for Apple's cloud services that conglomerates a lot of stuff that other web services companies have done for some time.
So when you wake up your iPhone and it tells you "Your iCloud storage is almost full", what does it really mean? Should I disable this altogether? There are different scenarios where you use iCloud without even noticing. The one that triggers this alert message every day has to do with the personal content that is is saved automatically for you as you use your device. The information stored ranges from pictures taken with your iPhone, settings and preferences for apps as well as documents and other data third-party app create.
Where is iCloud?
Even if you don't remember, you probably enabled this cloud backup feature when you first activated your iOS device. As part of the initial set up, you were asked to log in using your Apple ID account (the same you use to purchase on iTunes). To get you started you get an allowance of 5 Gb for free, with annual plans for people that require more space than that. So basically the error message "Your iCloud storage is almost full" is telling you that there isn't much left of those free 5 Gb you get to backup your iPhone content in the cloud.
There are two possible approaches here. You pony up for more storage subscribing to a yearly plan or remove some of the stuff you're saving that is taking up space. Even if you decide to upgrade to a paid iCloud plan, getting rid of the unnecessary junk sounds like a good idea anyway.
Just like it the general storage of your iOS device, you can select and delete individual items directly from the settings menu. Handy guide here in case what you're running out is of physical space. You can access this from the Settings app, iCloud, scroll down to Storage & Backup and finally Manage Storage. Alternatively you can also access this menu via General > Usage > iCloud.
On the 'Manage Storage' view will see the devices linked to this account as well as documents creates on your desktop and saved to iCloud rather than the local drive. Let's focus on the devices available. Top trick: if you see a device you don't use listed there, you can remove it tapping on its name and the button Delete Backup.
Now you're inside the Info pane of your device, you'll see Latest Backup date and the size of the backup. The idea is to reduce it as much as possible to keep your iCloud storage well trimmed and avoiding paying for storage you don't really need.
The usual suspect is the backup copies of your Camera Roll. All those shots are taking up space both on the device and keeping a copy of them on the cloud. Given the emotional attachment we have to our photos, you probably want to keep keeping security copies online just in case.
Before you carry on, you should be aware that another feature called Photo Streams is also backing up the last 1,000 photos (not videos for now) automatically when enabled. It's worth noting that Photo Streams are not part of the iCloud allowance, so you might want to disable the iCloud backup of photos if you plug your device to a computer regularly and backup manually. At least you have the reassurance that the most recent 1,000 will be safe.
Without a better name to illustrate, the next apps using iCloud storage are those used to read and play content. These can go from iBooks, Instapaper, Pocket, Evernote, Podcast players, PDF editors or comic readers. You will have to assess if these apps have a service behind that is storing the documents for you and assess if you really need to duplicate this. The ebooks on your iCloud can be downloaded in the future from the iBooks Store directly without having to reinstall an old backup copy of your phone.
iCloud email messages
If you are also using iCloud as a service provider for your email (I told you the iCloud brand covers a lot of web services) your storage is also affected by the amount of email messages you are keeping. If you aren't doing so, you can delete those emails with large attachments that you have downloaded somewhere and don't need.
Hopefully considering what you want to keep and discard you can free up some some space from your iCloud account. Having two or more devices linked with the same account will use up the 5 Gb very quickly and remember you need to follow this process on each device.