Blockchain cryptography uses pairs of private and public keys. The keys themselves are strings of characters that are cryptographically intertwined, meaning a private key will always be associated to the same public key and vice versa. The public key can be deducted from the private key, but not the other way around.
The private key enables unrestricted control over funds stored in the associated wallet - also referred to as account - and should therefore be kept safe. The public key is used as an address to identify a wallet. A simple comparison would be with a mobile phone, where the private key is the SIM card and the public key is the telephone number.