Can I use this with my friends using Gmail or Hotmail?
Yes! However, the messages they send to you will not be encrypted. You can send messages to outside users as well. For outgoing messages to non-ProtonMail users, you can choose to send them unencrypted or encrypted. Unencrypted messages are sent just like any other email.
PM → Gmail
PM → PM
Gmail → PM
You can send a friend using Gmail an encrypted message by selecting Encrypt for Outside Users. You will be prompted to set a message password to encrypt the message - this is the password you give to your friend. When your friend receives your encrypted message, they simply click on a link in the email, enter the password that you've provided, and the message is instantly decrypted for them!
On the other hand, messages exchanged between ProtonMail users are automatically encrypted without requiring passwords for individual messages.