Private key: spending authority
A private key is secret data used by wallet software to authorize a spend. Whoever obtains the necessary signing and recovery material may be able to spend the outputs it controls. Keep it private and follow your wallet’s official backup guidance.
Public key: derived verification data
A wallet derives a public key from a private key. Bitcoin scripts can use public keys or related hashes to set spending conditions. A public key is not the same thing as the private key that controls it.
Address: a payment destination
A Bitcoin address is a human-friendly encoding associated with a payment condition. Different address types encode different data, and not every address is simply a public key. Use the exact destination your wallet displays and verify it before sending.
Worked example
Think of a wallet generating a key pair: the private key is used by the wallet to sign, and the corresponding public information helps others verify a spend. The wallet can provide a receiving address without giving the sender the private key. Sharing a receiving address does not authorize the sender to spend your funds.
Common mix-up
“Public” does not mean every identifier provides the same privacy. Reusing addresses or linking them to your identity can make activity easier to associate. Avoid sharing private keys or recovery phrases, even if a person claims they only need to verify an address.