It's no secret that the NSA is collecting communications data, including email messages. Fortunately, it is possible to make your email messages unreadable to anyone but the desired recipient using the OpenPGP encryption standard.
Email encryption works by using "public" and "private" keys. The public key is meant to be shared with everyone, since this is the key that will actually do the encrypting of the message. The private key, which is never to be given out to anyone, will decrypt the messages. So if you want to send someone an encrypted message, you'll need to use their public key. If someone sends you an encrypted message, you'll need your private key along with your passphrase to read it.
In order to set this up, you'll need a way to create your key pair. For Windows users, Leo recommends Cryptophane. For the Mac, download GPGtools. This software will manage the keys of your recipients along with create a public and private key for yourself.
On the Mac, GPGtools includes a plugin for Apple Mail in OS X that uses the OpenPGP standard for encrypting and decrypting email. For Windows, Microsoft has help documents for setting up email encryption in Outlook 2003, Outlook 2007, Outlook 2010, and Outlook 2013.
Web-based email typically doesn't support sending and receiving encrypted email, but Hushmail.com is specifically designed to do this. There's also a browser plugin called Mailvelope that adds email encryption to webmail such as gmail, Yahoo! Mail, Outlook and more.
There's also a program called Crypto.cat which is an instant messenger client that uses this same public/private key technology to let you chat privately.
To watch Leo's explanation of email encryption, click here.
Watch episode 50 of Know How for more on encrypting email with PGP.