Seed

A mnenemonic seed is commonly used to sync two different devices. It’s also used to restore a lost or deleted software (eg. wallet). Some wallets grant access simply by using a long seed as a password and integrate the private key into the software. With this kind of software it’s best to choose the one which lets you export the private key manually or automatically. The seed as well should always be printed, at least once and kept in a secure place!