It's often convenient to put confidential information in an email - names, addresses, bank details, credit card numbers - but we really wouldn't recommend it. By default your message will be saved in plain text, then travel across multiple computers to reach its destination, so the contents can be easily intercepted on the way.
Install Comodo SecureEmail, though, and you're able to encrypt and sign outgoing messages with your own digital email certificate, which means only the recipients will be able to read them.
The program works with just about any email client, and of course most of these can already encrypt messages anyway, but SecureEmail does make the process easier. It automatically gets you a free (for personal, non-commercial use) certificate if you need one, for instance. It can then automatically encrypt emails as they're sent. And you don't have the usual hassles of exchanging certificates first, either - just send an encrypted message, and your contact can view it immediately, either by installing Comodo SecureEmail, or using an online reader.