There is a many-to-many relationship between issues and interactions. During the course of one interaction, a customer may bring up any number of issues ("My credit card shows a charge for $7.99, but I thought this ant farm was only supposed to cost $6.99 AND, while I have you on the phone, I'd like to mention that delivery took three days instead of the promised two."). Furthermore, if an issue is not resolved the first time it is brought up, it might span any number of interactions until it is finally closed.
Issues can be categorized either for reporting purposes or so that different departments of your company can handle the issues. Open issues are linked to from the front page of the customer service submodule to attract attention. Whenever email is sent to the customer (either automatically or by a customer service rep), an issue is created (or added to, if it is based on a previous issue). This is so that a complete interaction history containing all correspondence to and from the customer can be maintained. All issues created due to automatic emails are closed immediately so that they don't get in the way of other issues.
Small note: the intersection between an issue and an interaction is called an "action" (i.e., the part of a specific interaction that deals with a specific issue). This rarely comes up.
Whenever you record a new interaction, you are asked to enter as much information as you can gather (or feel comfortable gathering) about the user. The system then tries to match this person up with either registered users or unregistered people who have had interacted previously with customer service. If no match can be made, a new "user identification record" is created.
Each time you view a user identification record, the system sees if it can match that person up with a registered user of the system (in case they have registered in the meantime).
If you find yourself using the same phrases over and over again when you respond to customers' emails, the Canned Response System will be useful to you. You can enter your commonly used phrases once, and then whenever you send email you'll be able to automatically insert any number of these phrases.
If you want to send email to customers in bulk, then use the Spam System. You can spam users based on what products they've bought, what products they've even looked at, by when they've last visited, by how much they've spent at your site, by which mailing lists they're signed up for. If you're spamming customers that you like, you can issue them all gift certificates at the same time.
Your email text is also sent through a spell checker before it is sent to the customer.