You will see "Types" and "Categories" referenced throughout the documentation.
"Types" is a group that cannot be defined by the end user and is used throughout OptiPub. For example, "MessageTypes" is the type of messages that your instance of OptiPub is allowed to send such as "Email Article," "SMS," "Email Free Style," and many others. These are managed by an OptiPub administrator and can not be managed by you.
"Categories" are similar to "Types" except that the end user is able to fully manage these. "Categories" are used to organize your OptiPub instance to fulfill your grouping needs.