Tagging is similar to flagging, except that you can create and apply multiple tags to a single item in order to classify it under several categories simultaneously. After creating a tag, you can apply it to conversations, messages, or contacts.
Drag and drop the tag from the left-hand Overview pane onto the item on the Content pane.
Right-click the item from the Content pane.
Choose Tag Message (or Tag Conversation or Tag Contact, etc.).
Choose the tag name from the list that appears. Only tags not already applied to the selected item are listed.
Right-click the item from the list and choose the Tag menu option that appears.
Choose New Tag.
Enter a name for the tag and click OK.
Right-click the item from the list and choose the Tag menu option that appears.
Choose Remove Tag. If the item has multiple tags, you can choose to remove one or all tags.
Select all the items to be tagged using Control-Click or Shift-Click.
Right-click over any one of the selected items and choose the Tag menu option that appears.
Choose the tag to add or remove.
Click a tag in the Overview pane.

Alternatively, you can search for tags. In the Search field enter the query as tag: <"tag name"> and click Search. A query of tag:Purple would show all items that had a tag named Purple applied.
