Mail (Mountain Lion): Flag messages for action
You can flag messages that you want to track or follow up on later.
If you use iCloud Documents & Data, your flag names are available on your other Mac computers that use Documents & Data and have OS X Mountain Lion installed.
Select one or more messages, click the Flag icon in the Mail toolbar, and then choose a flag.
A flag symbol appears in or next to the messages.
Select one or more messages, click the Flag icon in the Mail toolbar, and then choose Clear Flag.
Flags appear in the sidebar in the Reminders section. If you have used more than one flag, the flags are listed by color. To change the name of a flag, double-click it and then type a new name. For example, double-click Red, and then type Urgent.