Glossary
What is a public key?
In one sentence
A public key is derived from your private key and is safe to share — it is what lets others verify your transactions and send you funds.
Public-key cryptography uses a matched pair. The private key is secret; the public key is derived from it and can be published without risk. Your wallet address is generated from the public key.
The pairing does two useful things. Anyone can encrypt information to your public key that only your private key can unlock, and — more relevant here — you can sign a message with your private key that anyone can verify using your public key. Crypto transactions rely on the second.
Sharing a public key or address reveals nothing that endangers your funds. It does, however, reveal your transaction history, because blockchains are public. Anyone with your address can see every payment in and out and the current balance, which is why privacy-conscious users avoid reusing a single address for everything.
For example
Giving someone your address so they can pay you is completely safe. Giving them your private key gives away everything.