What is ProtonMail?
ProtonMail is an email service hosted in Switzerland, run by some serious nerds from CERN, where your whole inbox is encrypted with a secondary password so that no one, not even ProtonMail themselves, can access it. Privacy comes first for services like these. Even if e.g. law inforcement were to ask for your mailbox as stored at ProtonMail, it would be useless as they could only hand over the encrypted files, and only you know the key.
Messages between you and other ProtonMail users are encrypted transparently, but you can also sent encrypted email to people with regular email accounts. All they will receive is a link, and you have to exchange a password with them by other, secure, means.
Are there other services like ProtonMail?
Yes, there are at least two others: Lavaboom (waiting list only) and Tutanota (open to the public), both hosted in Germany.
Are the mods here affiliated with ProtonMail? Is this an official subverse?