It is a super-network consisting of thousands of networks all linked together. When you send an email, it travels though many different computer systems which route the message to its destination. At any point in its journey, it is possible for someone to intercept it. Any security you have on the internet is due to the fact that hundreds of millions of messages are sent every day. The odds of yours being singled out are small, just because there are so many others.
Encryption is one solution.
Encoded messages cannot be deciphered by anyone except the recipient. This includes things like credit card numbers, social security numbers, driver's license numbers or other pieces of information that could be used to steal from or defraud your visitors. Get a secure server for these transactions. Adgrafix has a very reasonable secure server solution if you use them to host your website. Look into some of the newer forms of payment, such as CyberCash and First Virtual.