Categories allow you to easily sort your mail in the fly, making messages from specific addresses, certain subjects, and destinations with a label and color. These labels translate to programs that can support categories, such as Outlook and Entourage. Labels can be added manually to a message, or can be done through inbox rules for new incoming messages.
Click on the in the top right of your message.
In the Color dropdown, you may choose the color of the label for this message.
In the Category Name field, enter a label for this category.
Click on OK
Once you are done managing your categories, press OK to continue.
You may set up Inbox Rules to automatically categorize incoming messages.
To display only messages that fall in a certain category, click on the Filter button, go to Category, then choose the categories you wish to view. Click Apply to filter out all other messages.