You can assign keywords to your attachments using tags. Unlike categories, tags have no hierarchy, meaning there’s no relationship from one tag to another.

What’s the difference between categories and tags? Normally, tags are ad-hoc keywords that identify important information about your attachment (names, subjects, etc.) that may or may not apply to other attachments, while categories are pre-determined sections. If you think of your site like a book, the categories are like the Table of Contents and the tags are like the terms in the index.

You can change the display of this screen using the Screen Options tab to set how many items are displayed per screen and to display/hide columns in the table.

When adding a new tag on this screen, you’ll fill in the following fields:

• Name - The name is how it appears on your site.
• Slug - The “slug” is the URL-friendly version of the name. It is usually all lowercase and contains only letters, numbers and hyphens.
• Description - The description is not prominent by default; however, some themes may display it.

The “Attachments” column at the right of the table gives you the number of attachments associated with each tag. You can click on the number to get a list of all the attachments with that tag. The heading on the list page(s) will display the tag value you’ve selected.

For more information:

Documentation on Tags

MLA Support Forum