What is data backup? A plain-English guide

Data backup is the practice of copying your files, systems, or databases to a separate location so you can restore them if the originals are lost, damaged, or locked away. Hard drives fail, laptops get stolen, files get deleted by mistake, and ransomware can encrypt everything on a network in minutes. A backup is what turns those events from a disaster into an inconvenience.

Why data backup matters

Most data loss isn't dramatic. It's a spilled drink, an accidental delete, a failed update, or a drive that simply stops spinning. For businesses, the stakes are higher: customer records, invoices, and project files can be impossible to recreate, and downtime costs money every hour.

Common causes of data loss include:

• Hardware failure: every hard drive and SSD eventually wears out.
• Human error: deleting or overwriting the wrong file is one of the most common causes of loss.
• Theft and loss: laptops and phones go missing, taking local files with them.
• Ransomware and malware: attackers encrypt or delete data and demand payment.
• Disasters: fire, flood, and power surges can destroy every device in one location.

How data backup works

Backup software scans the files or systems you choose and copies them to a backup destination, such as an external drive, a network-attached storage (NAS) device, or a cloud backup service. After the first full copy, most tools only send what has changed, which keeps later backups fast.

Good backup tools also keep versions. If a file is changed, corrupted, or encrypted by ransomware, you can go back to an earlier copy from before the problem started. How long those versions are kept is called the retention period.

The main types of backup

Full backup

A full backup copies everything you've selected every time it runs. It's the simplest to restore from, but it takes the most time and storage.

Incremental backup

An incremental backup copies only what changed since the last backup of any kind. It's fast and space-efficient, and it's how most modern cloud backup services work after the first upload.

Differential backup

A differential backup copies everything that changed since the last full backup. Restores are simpler than with incrementals, but each differential grows until the next full backup.

Image (system) backup

An image backup captures an entire drive, including the operating system, apps, and settings. It lets you rebuild a computer or server quickly after a failure, rather than reinstalling everything by hand.

Where to store backups

Each destination has strengths, which is why most good strategies use more than one:

• External drives are inexpensive and fast to restore from, but they sit in the same place as your computer.
• NAS devices back up several computers on a home or office network.
• Cloud backup stores copies offsite in a provider's data centers, protecting you from local disasters and theft.

Combining local and cloud copies is the idea behind the 3-2-1 backup rule: three copies of your data, on two types of storage, with one copy offsite.

Backup vs. sync: an important difference

File sync services keep the same files on all your devices. That's convenient, but sync also copies mistakes: delete a file on your laptop and it disappears everywhere. A true backup keeps independent, versioned copies. We cover this in detail in cloud backup vs. cloud storage.

How to start backing up today

1. List what matters. Documents, photos, email, financial records, and anything you couldn't recreate.
2. Pick an automatic method. Backups you have to remember to run eventually stop happening.
3. Add an offsite copy. A cloud backup service is the easiest way to keep a copy away from home or the office.
4. Turn on encryption and multi-factor authentication for any backup account.
5. Test a restore. Recover a few files now, so you know the process works before you need it.

If you're comparing services, our 10-point checklist for choosing a cloud backup service walks through the features that matter most.