Key pair, public key and private key, refers to Public Key Cryptography. In this scheme, the public key is distributed to everyone, while the private key is kept safely secret.
Email sender uses your public key to encrypt the email message and you use the private key to decrypt it. Since you are the only one with the private key (safeguarded), you are the only one who is able to decrypt the email message.
Secret key refers to Secret Key Cryptography. In this case, the secret key is used to both encrypt and decrypt the data. Since this method is fast, it is used for encrypting attachments over a certain size, defined by you. The secret key is transferred using the public key cryptography in an attachment.