Ledger devices work following BIP39 blockchain protocol, so it manages a seed that generates your private keys and your wallets. This way to generate and backup your wallets is not a Ledger specificity, it is a well-known secure standard, which makes your Ledger device compatible with the other BIP39 wallets. You can know more about it here.
Ledger devices generate their own seed, always constituted of 24 words, coming from the BIP39 dictionary. This seed is generated when you initialize your device. Each device can generate several seeds, but can only manage one seed at a time on one device.
HOW TO STORE YOUR BACK UP
You are the only one who ever saw this list of 24 words. It is only displayed once, when you configure your device. For security reasons, it will never be displayed again, take care to copy each word really carefully.
These 24 words are not stored by anyone but you, not in a server, nor anywhere. Even Ledger can not access them, in any way.
You must copy them on the Recovery Sheet provided in your Ledger box, in the correct numerical order. Be careful, if you make 1 or several mistakes or typos, you won't be able to recover your wallet if you need to. You could lose all your funds.
HOW TO IMPORT A SEED FROM ANOTHER WALLET
You can also import your own seed if you already have one. For example, you can import a seed from another Ledger device, so both your devices would become clones.
Be aware that if you import a seed from a software wallet, you can not be absolutely sure it has never been exposed or read by a malware before it is protected by the Secure Element of your Ledger device.
On a Ledger Nano
On a Ledger HW.1
You can import a 24 words seed only.
On a Ledger Nano S
On a Ledger Blue
You can import 3 kinds of seeds: