They encrypt all mail messages and they are at Switzerland, where data protection laws are very restrictive. They also assure end-to-end encryption, so the data is already encrypted by the time it reaches their servers. They cannot decrypt the encrypted messages and as result, and so, they cannot share them with third parties.
You can send mails to other non-protonmail users and encrypt the message by giving a password to your interlocutor, and the mail you send it's a mere link to a webpage where the other side must enter that password in order to see the message. This webpage will be deleted in the time you specify.
They, as duckduckgo, has their motto: We don't track our users.
Seems a nice service, I'm using it from a month or so and it works pretty fine, but we're so few people that at the moment, I've sent a few encrypted messages...