Tags are labels you can use to attach a detail or category to a specific item.
You can then use this tag to search for these items, look through special tag folders or just use the colors to make your data more organized.

You can manage tags in Menu > Tags section.
You can manage tags for all items in eM Client - Mails, Events, Tasks, Contacts and Notes.

When you create a tag you can give it a name, assign a color and even a keyboard shortcut.

You can then specify if the tag can be used for messages, events, tasks, contacts or notes or any combination of these.

If your items are synchronized with a server that supports the tags feature, they will be synchronized across your devices. Otherwise, the tags will be kept locally.

Gmail labels are treated as Tags in eM Client.

Note: Tags have been called Categories in eM Client 7 and all older versions of eM Client.

Read more in the Documentation: About Tags