Tags vs. categories
Tags and categories are two options to organize topics within a group. While these options are similar in their purpose, they have some differences in how they are surfaced to members and only one of these features can be enabled in your group. The remainder of this article describes these two features to help you pick which one you might use.
Tags are special words used to associate topics with subject areas. Tags are created by administrators (or members with the "enter free form tags" permission) for the group and then applied by members to individual topics. For example, an administrator of a group called "Smart Phones" might create a tag called "android." Group members can assign this tag to topics in the group to identify the subject as relating to "android." Other group members can search on topics with the tag "android" to find related topics. Use tags if you just want a way to search for topics on a subject area.
Categories are similar to tags in that they are used to organize topics into subject areas. However, categories are more formal in that enabling categories causes the first page in a group to be a list of categories requiring members to first drill down to a subject area before entering into a discussion or posting a question. Use categories if you want to give some structure to a large or busy forum.