NASA Category Search is a subject-based library of NASA content organized into categories and subcategories. For example, you can see a listing of sites related to NASA organizations in Organizations category.
The category search can be used when you want to narrow down your searches from a broader scope. For example, if you are interested in information from a particular NASA center (say JPL), you can go to the category Jet Propulsion Laboratory (JPL) under NASA Centers and enter your search query there. It will return results from the JPL category only.
The Category Search can also be used to get more information about the subject you are interested in. For example, if you are interested in knowing about the missions to Mars, you could click on the category NASA Missions and Projects, then Solar System Explorations, then Missions to Mars, where you would find information about the missions to Mars. You could also find information about missions to other planets, e.g., Jupiter or Saturn.