Similar to follow up flags, Outlook categories can be used mark and group e-mails. Categories are global in Outlook, in that they can be used on all different types of Outlook items: e-mails, calendar items, contacts, tasks, notes, and journal items.
Suppose you're working on a large project at work and want to mark all Outlook items associated with this project. You can create a category and then categorize all Outlook items that are related to this project. In the different areas of Outlook (Mail, Calendar, Contacts, Tasks, etc.), you can group and view items by category.
