With the help of keywords you will greatly enhance the chances of selling your photos. Consider this process, a sort of mini SEO (search engine optimization). Without tags the website's built in search engine can't show an accurate result. If you get your keywords right, you greatly improve the chances of your photo being shown first in the search results. As with any other aspect of stock photography, look at your competition and start by analyzing popular tags.
Start with a keyword list that describes your photo:
Query these keywords on stock photo websites and see what comes up. If you found a photo that is similar to yours check that photo's tags. Avoid using generic keywords such as dog or flower, because stock websites are saturated with these keywords, instead try to think of more specific tags such as "dobermann puppy". If your photo illustrates people, again be specific and make sure you mention the mood of the people depicted: "Happy woman". Descriptions
Usually, 2 to 3 sentences is best for the description of a photo. If your photo illustrates a location make sure you mention it in your description (or in the photo title), and be as specific as you can.
Don't write long descriptions, because it's a fact that users are not reading website content, they scan it. Try to capture the essence of the things that your photo illustrates such as objects on the photo, the mood of the photo, and location when necessary.