We send emails and attachments all the time.. we don't really think about the security of these attachments, we just assume that only the eyes of the sender and the receiver will see them. You may even have an encrypted or secure email connection, but realize that email transmission is only as secure as the weakest layer in the transport of the email. This layer may and most likely will be completely out of the sender or receiver's control.
One way we can help to combat this is to "zip" or compress files/folders and apply a password to the zipped file (archive) before it gets attached and sent. Then, you could attach the password protected zip file and send the password in a separate email (or better yet, call the recipient and TELL them what you're going to be using for the password).
So, how do we password protect a zip file? Let's see if we can explain it step by step using a nice free utility called 7zip.