It's essentially like encrypted Hotmail. From their site...

"Hushmail is a Web-based e-mail service that lets you send and receive email in total security. Hushmail messages, and their attachments, are encrypted using Open PGP standard algorithms. These algorithms, combined with Hushmail's unique key management system, offer users unrivalled levels of security. Hushmail's security is end-to-end; messages are encrypted before leaving the sender's computer and remain encrypted until after they arrive on the recipient's machine, where the contents are automatically decrypted.
Hushmail's encryption works automatically, transparently and seamlessly, requiring no specialized computer skills or knowledge. Encrypting a message is as simple as clicking a mouse."