In order to protect emails and other important information sent over the Internet, the only secure way is to encrypt them.  Many encryption software exist, but only one is really secure:  PGP.  PGP (Pretty Good Privacy) is a powerful, free cryptography package.  PGP lets people exchange files in a private, encrypted format, and also provides message authenticity.  PGP is called a public key system because it can be made widely available and is used to encrypt messages.  The keys are actually a digital signature, a small file with a stream of uniquely generated numbers.  The public key is widely distributed to any correspondent and the private key is guarded with secrecy.  An encrypted message in PGP is one which is scrambled in a complex way to make it unreadable to anyone.  You can use you private key and someone else's public key to generate a message that can only be unscrambled by the person holding your public key and their own private key.  This allows you to easily exchange encrypted messages with anyone you have exchanged public keys with.  The reason PGP is considered a high grade encryption program is that it uses encryption keys longer than 48 bit of data to encode the contents of a file.  PGP has the capability of generating 2048 bit keys, which render a file useless unless one possesses the original key that was used to encrypt the file.

Example of an encrypted file:
 

Once PGP encrypts the information, the following file results:
 
 
 

In fact, PGP so thoroughly encrypts its data that theoretically it would take numerous supercomputers running simultaneously for millions of years on end before the key could be cracked. Because of this powerful encryption algorithm, the U.S. regulates the export of PGP to prevent international entities from using it.
 

All of this information was taken form The Concern for Privacy on the Internet, Chaper 3:  Counterattack

You can also view this video tutorial of encryption.