One of the most powerful features in Lightroom is its keywording capabilities. You could argue that this feature alone is a great reason to buy the application! Some digital asset management aficionados have argued that Lightroom just isn't powerful enough to qualify as your main DAM application. That's just not so!
Contained within Lightroom's keyword feature is the ability to create hierarchical keyword structures. Included in this is capability to add non-exporting categories and keyword synonyms. Let's examine the keyword hierarchy feature.
Say you have an image of a dog. One logical keyword for this image would be dog. But you could also tag it with mammal and/or animal. If you did use all three keywords on the image then you would see three separate keywords in your list. The next image of a dog you came across would require you to tag the image with three keywords. But if you had arranged your keywords into a hierarchy you could apply only one and get three!
There are two ways to do this quickly in Lightroom without using the Create Keyword Tag menu. In the Keywording panel click in the field that reads Click here to add keywords. Now, depending on how you think, enter the hierarchy all at once.
The first method is dog>mammal>animal. This reads dog within mammal within animal. Press enter and only dog will appear in the keyword box above. If, however, you change the view to either Keywords & Containing Keywords or Will Export you will see all three keywords animal, dog, mammal. (Lightroom alphabetizes all keywords.) Now all you have to do with the next dog image you come across is tag it with the keyword dog and all three keywords will be attached!
If you look down in your Keyword List panel you will see the hierarchy you just created.
If your brain works in the other direction you can enter animal|mammal|dog. This reads animal containing mammal containing dog. So whichever direction works for you I'd encourage you to start creating keyword hierarchies.