Keywords are the words used by search engines to find your images. Wherever you have you images stored, be they on your own computer, on a web site or in an image library, the way for you and your customers to find them is through the keywords associated with the image.
In anyones language, that makes the keywords associated with the image extremely important, and so much more so if you are hoping for image buyers to find them! DAM (Digital Asset Management) software relies on them too, assisting in cataloging large collections.
Keywords are kept with an image in a sidecar file known as an XMP file along with all the other data about the image that is stored by the camera metadata system such as the shutter speed, aperture, date and time etc. and is in a standard form known as IPTC.
However, at present, the camera does not know what the image is of, so the keyword field in the metadata is left blank by the camera and you, the photographer, needs to fill it in to a suitable level for use, either by you or subsequent users of the image.
Modern DAM software, such as Aperture (Mac) or Lightroom (PC) allow you to add keywords during the import process. This does, however, tend to slow this process down considerably and is only really useful when importing large numbers of images of the same subject or from the same shoot. So, for example, a day wandering around the Capital shooting landmarks could perhaps have the keywords 'London' 'England' and perhaps the time of year like 'Spring' or 'Autumn' included as generic words to the whole shoot.
These type of programs however, do provide the ability to keyword after import, so, once they are in your catalogue it is much quicker to keyword images individually by selecting the similars and adding the keywords in the appropriate boxes, so that 'Trafalgar' and 'Square' can be added to the 'London', 'England' and 'Spring' on the twenty or whatever images where that is appropriate. Reselect the half dozen images that contain the Lions and add 'Nelsons' 'Column' and 'lion' and so on until each image has it's own unique set of keywords.
The same thing is possible in editing programs such as the Photoshop series of software applications, where the dropdown menu gives an option >File Info. Click on it to bring up a window where you can add and edit a Caption, Description and keywords to the individual files, as well as any copyright information and contact details.
All this information then stays with the image file, allowing other systems to read it, such as those employed by image libraries and their search engines.
A word of warning here though, most image libraries will provide guidelines for keywording and these do need to be followed carefully to get your images seen on their system. Inappropriate words, used to get your images seen more, but with no relevance to the actual image, will be picked up by their seach algorithms and downgrade your images in their search engines. This is because it annoys their buyers when they enter a seach term, only to find hundreds of images of no interest to them popping up.
Rudyard Kipling once said "I have six honest, serving men. They taught me all I knew. Their names are What, and Where, and When. And How, and Why and Who!"
It is good to follow this advice when writing keywords (as well as in many other places), sticking to the facts about what the image is, or contains without over embelising the wording. It will pay dividends in the long run.