Gmail introduced new Drag & Drop interface. This interface helps you to manage labels to your E-mails.
This interface allows you to add/remove labels easily. Also you can drag drop lebels from sidebar to an E-mail.
You can drag and drop labels to an opened email or to title of email. Lables are same thing as tags , which you often add to many things.
Additionally if you want to configure Gmail for Outlook Express or any other POP3 client here is an guide: Access Gmail From Microsoft Outlook