You can think of a private key like the secret coordinates for locating your cryptocurrency, which means that whoever knows your private key has control over your coins. In general, it is a very long string of numbers and letters that acts as the password to your cryptocurrency wallet and it’s from this number that your wallet gets its power to send your coins to other people.
The private key is also used to generate your public address and it’s something you give out to people who want to send you cryptocurrency. However, there’s no way figure out what the private key is just by examining a public address, even though the public address is generated through the private key.
To sum it up, the wallet’s core function is the creation, storage and use of the private key, which means that the wallet automates the complex cryptography for you.
HD wallets, or hierarchical deterministic wallets, were created as cryptocurrency wallets evolved. HD wallets generate an initial phrase known as a seed or mnemonic phrase and this seed is a string of common words which you can memorize instead of the long confusing private key.
Later on, you can enter these words if your wallet gets destroyed or stolen in order to reconstruct the private key. Additionally, all of the transactions sent to addresses created by the same seed will be part of the same wallet because an HD wallet can memorize many public addresses from the same seed.
These private keys and seeds must be kept secret and safe because they have complete power over your coins. The cryptocurrency that your wallet’s private key or seed controls could be irretrievably lost if you fail to protect it.
An HD wallet on the other hand will supply you with a seed phrase. This phrase contains up to 24 words that you should write down in a safe place.
The public key is the address that anyone can use to send you the cryptocurrency. On the other hand, the private key is what you will use to send cryptocurrency to anyone else. ONLY you should know what your private key is, remember that, and under no circumstances should you ever lose your private key. Otherwise anyone can use your wallet to send your money to any other addresses.