Categories help to organise media on Commons, making it easier for others to find and reuse media.
Categories and Galleries
Other people need to find your file in order to use it to illustrate articles in other Wikimedia projects, so it is crucial to add your media files to specific categories. Every image must be in at least one classification category; otherwise your valuable media files won't get used as much as they could be.
If you wish, you may also add them to specific gallery articles on Commons. An example of a gallery page could be the image gallery of the planet Mars.
You can tell if a page is an article or a category by looking at its name. Pages without a prefix are article pages, pages with a "Category:" prefix are categories and pages with "File:" are File pages (images, sound or video). This concept (implemented with these prefixes) is called "namespaces". That way you can easily identify different content types by their namespace prefix.
Adding images to categories
Adding an image to a category is done like adding a Wikipedia article to a category. Simply place something like the following example code at the image page itself:
Be as specific as possible. Don't add an image to an overcrowded root category. In order to find a right category for your image, the category tree function will help you a lot: just go to a general category, such as Category:Countries and then follow the subcategories until you find the most specific one that describes your image.
Adding images to galleries
To add an image to a gallery, first upload the image, then navigate to the category page the gallery is in and click on "edit". In the edit box you will find the gallery's images listed between the opening and closing Gallery tags:
<gallery> Image:Mars Valles Marineris.jpeg|Valles Marineris on Mars Image:Mars Hubble.jpg|Mars seen by the Hubble Space Telescope, Realistic Colors </gallery>
Add the Wiki-file-link for your image between the opening and closing gallery tags without the brackets, so that it's in the same format as the other image files:
Image:Your photo name.jpg|A brief description
In addition to subject categories, it is helpful to add categories indicating software or equipment used in making the file, such as: