Keywords are labels that you can create and assign to each of the photos in your Photo Album or photos in your Photo Library. Keywords can be useful when you are searching through your account for specific photos. You can have different keywords assigned for the Photos in your Photo Album than what appears for individual Photos in your Photo Library.
All of your keywords are listed in the "Manage Keywords" dialog. You can access this dialog in your Photo Library. From this dialog, you can add new keywords, edit existing keywords, and delete keywords. To learn how to access the "Manage Keywords" dialog see the image below:
After you click on the blue "Manage Keywords" link a pop-up dialog will appear listing all of your keywords. Follow the steps in the image below to delete keywords:
When you delete a keyword it will be deleted from all photos that this keyword has been applied to. This included keywords applied to photos in your Photo Library as well as keywords applied to photos in your Photo Albums.