Data Categories are shared by Knowledge and Chatter Answers.

Data Categories are extremely powerful. 

• The both categorise AND restrict content.

Read and understand this article http://help.salesforce.com/apex/HTViewHelpDoc?id=category_visibility_whatis.htm&language=en_US

Read and understand the limits https://help.salesforce.com/apex/HTViewHelpDoc?id=limits_general.htm&language=en_US 

• You can only have 3 active Data Category Groups and 100 Categories in 5 levels within each group. 
• I think 3 groups is quite limiting, but I understand the complexity of them.

Permissions

Permissions can be set by:

• Role
  • This is useful for a hierarchical permission set mimicking the role hierarchy. 
    • Eg Territory based Knowledge articles for internal users.
• Permission Sets
• Profiles

Recommended Setup

Structure

• Make your Data Category Groups quite generic. 
• Use the hierarchy to be more granular.

Permissions

• Set your Default Data Category Visibility settings to None, or to only those that would absolutely be visible to ALL people, including internal users and all external users including community users and general public. 
• Use Permission Sets to grant Visibility.