You create a pair of keys that are mathematically related to each other, one is public and the other is private. Never share your private key with anyone. [It is used to decrypt messages to you.] Give your public key to your friends. [It is used to encrypt messages to you.] Collect public keys from your friends. Use their public keys to encrypt messages to them. They use your public key to encrypt messages to you. You use your private key to decrypt messages. [Meaning you have to select your key in the interface.]