A private key is a key that provides access to a wallet, similar to a password.

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A wallet is on the blockchain and is used to hold coins. This is similar to a bank account with an important difference, which is that the wallet on the blockchain is public. The address of the wallet (public key) is visible to everyone, as are the contents of the wallet. The private key, however, is only for the owner of a wallet and thus must be kept securely.

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A private key is almost never handled by the user, instead the user is usually given a seed phrase or recovery phrase that encodes the same information as a private key.