Email (e-mail, electronic mail) is a popular form of modern communication, whereby message are sent via the Internet between parties.
The easiest way to use public key encryption with email is to install the Thunderbird email client with the Enigmail extension. You'll also need the GPG software package. It sets up fairly easily but may intimidate some people.
If you wish to access browser-based email and use PGP, you are best off setting up Thunderbird to access it via POP, IMAP, etc. If that is not possible, there's a browser extension called Mailvelope which will enable you to use PGP with gmail and some other email providers via a web browser.