When you encrypt a mail, you essentially encode it. But you also generate a "key" for the recipient to decode it at his end. Keys are also used to encode messages. That means you need to send the key separately to the recipient before sending the encoded mail. But what is a key?

A key is just a software file with some unique characters in it. Random numbers are used to generate this key. When you encode something, the encrypted file or mail is informed about the unique key. During decryption, the unique key is associated with the encrypted file, and then its contents are decoded-and you will be able to read the text once again.

Keywords are password protected, so you also need to exchange the password with the recipient. You will probably be aware that the key is supposed to be kept a secret. No one besides the recipient should get it. There are various kinds of encryption schemes that use one or two keys [known as Public and Private keys). The two common types of encoding are symmetric and asymmetric encoding. This is explained with the help of diagrams here.

Symmetric encryption uses a single key called a private key, while asymmetrical encryption (also known as the public-key procedure] works with two keys. The public key is used for encryption, while the private key is used for decryption
(at the recipient's end).