Select Match all or Match Any from the pull-down menu. (Example: Using the keywords cats dogs with Match Any selected will return images with cats OR dogs. Selecting Match All will only return images with cats AND dogs.)
NOTE: Match All/Any only applies when using more than one keyword.
Enter any keywords you'd like to filter out of your search into the not these words field. (Example: Enter cats in the keyword field and dog in the not these words field to find images with cats and NO dogs.)