What is a Public Key
A public key is the part of a key pair that you can safely share with anyone. It is used to encrypt messages that only the holder of the matching private key can decrypt. It is also used to verify that a digital signature was created by the holder of the private key. Because the public key can be distributed freely, it allows many people to send secure messages to one person.
What is a Private Key
A private key is the secret part of a key pair that must be protected and never shared. It is used to decrypt messages that were encrypted with the matching public key. It is also used to create digital signatures that prove you are the authentic sender of a message. If someone obtains your private key, they can read all your encrypted messages and forge your signature.
How Key Pairs Work Together
The public and private keys in a pair are mathematically connected through complex algorithms. When you encrypt something with a public key, only the corresponding private key can decrypt it. When you sign something with a private key, anyone with the public key can verify the signature is genuine. This relationship is designed so that knowing the public key does not reveal the private key.
Common Uses
Key pairs are used for several important security purposes. In secure email and messaging, they allow people to encrypt conversations. In online banking and commerce, they authenticate that you are who you claim to be. Websites use key pairs to establish secure HTTPS connections. Digital signatures created with key pairs prove that a document or software has not been altered and came from a trusted source.
Key Generation and Storage
Key pairs are generated using special cryptographic algorithms that ensure the keys are mathematically related but impossible to predict. The public key can be stored and shared anywhere, but the private key must be protected with strong security measures. Many people store private keys in encrypted files or hardware devices called security tokens. 
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