Eavesdropping, where your information remains intact, but its privacy is compromised. For example, someone could learn your credit card numbers, spy on an intimate conversation, or intercept classified information. Modification, where your original information is changed or replaced and then sent to the recipient. For example, someone could alter an order for goods, or change a person's resume. Impersonation, where your information passes to a person who poses as the intended recipient.
Impersonation can take two forms.
This type of impersonation is known as spoofing. The second type of impersonation is more subtle. You can prevent each of these threats if you use SSL for all communications. The information can still be intercepted, but if it is, it will be unreadable by a third party. The only information that can be discovered is that the two parties are communicating.