Outlook provides 20 default categories for you, but you may  want to create your own. Here's how:

1. Select one or more messages to assign to a category. 
2. Choose File, Categories (or right-click the items and choose Categories from the shortcut menu). 
3. Click in the Item(s) Belong To These Categories box and type the new category name (if you want to type more than one, separate the names with a comma). 
4. Click Add To List.
5. Click OK.