Definition
Cloud backup is a technology that automatically copies important data, like photos and documents from your phone or computer, over the internet to a remote server (a huge warehouse of computers). If you lose your device or it breaks, the backed-up data can be brought back.
Your data survives even if your device doesn't
Say you drop your phone in the ocean or lose it on the train. Paying for a new phone hurts less than losing years of photos and contacts. You can buy a new device. But memories and important work records can't be replaced once they're gone.
You could copy files to an external hard drive or a USB stick. But plugging in a cable every time is a hassle, and you might lose that drive too. With cloud backup turned on, your phone quietly adds copies to a safe place on the other side of the internet whenever it connects to Wi-Fi.
So even if your phone is totally wrecked, you can sign in to your account on a new phone and get your photos and settings back, close to how they were. Your device may break, but a copy of your data is kept safely somewhere else. One catch: some apps need their own separate backup.
How is it different from simple syncing?
Many people mix up photo "sync" and "backup," but they're not the same. Synchronization (sync) keeps data on several devices matching, like a mirror. So if you delete photos to free up space on your phone, they disappear right away from your tablet or laptop linked to the same cloud.
Backup, on the other hand, is a separate archive of copies. A backup that keeps multiple versions takes a snapshot of the past, like a photo. If you accidentally delete an important document or get a virus, a multi-version backup still holds the file as it was "last night" or "last week." But many backups, like a phone's device backup, keep only the latest copy. If you delete a file and the backup runs again, that file can vanish from the backup too.
With a version-history backup, you can turn back time and pull out the file from the moment you want. To bring back a file you lost by mistake or data that got corrupted, you need a real backup record, not just syncing.
A bit more precisely: keeping copies in many places
To be a bit more precise, big cloud services usually don't put your data on just one computer. To keep it safe from disasters like earthquakes or power outages, they copy it across many machines and data centers far apart from each other. But this kind of copying protects against hardware failure and disasters. It can't protect you from files you delete yourself or from deleting your account.
Data is also encrypted (scrambled so it can't be read without the key) both while it travels over the internet and while it sits on the server. So even if someone intercepts it in transit or steals a whole storage drive, the contents are hard to read. However, with most services the company holds the key, so it could open your data if it chose to. End-to-end encryption, where only you hold the key, can block even that. Depending on the service, it may be on by default, something you have to turn on, or not offered at all.
There are downsides, of course: you may pay a fee, and it's hard to download your data quickly where there's no internet. Still, it's widely used as one of the more trustworthy places to keep your data.
Common misconceptions
Since my photos are synced to the cloud, they'll stay there forever even if I delete them from my phone.
Sync makes your device and the server match exactly, so deleting a photo on your phone deletes it from the cloud too. If you delete something by mistake, you can only restore it from the trash ("Recently Deleted") within a set time. To keep it safe long term, you need a separate backup that isn't tied to sync, such as an external drive or another backup service.
Where you meet it
You switch to a new smartphone, sign in with your old account, and download your photos, contacts, and main settings all at once (some app data needs its own app-level backup).
Your laptop gets hit by ransomware (malicious software that locks your files). You roll your data back to a cloud backup from last week and recover it.
In one sentence
Cloud backup automatically copies your data to a safe remote server over the internet, so you can get it back anytime, even if your device breaks or disappears.