For blog galleries you have the option of specifying categories and tags. These are useful tools for organizing different blog galleries into related topics.
At the bottom of each blog gallery page is a tagline that displays the links to the gallery's associated categories and tags. Also on the main blog gallery page there is a category selection menu that your website viewers can use to filter through all the blog galleries.
The categories and tags basically do the same thing, but the difference is that the categories play a more dominant role since the category selection menu is on the main blog gallery page.
I would recommend using the categories for the broadest and most important classifications of your blog galleries, while the tags can be more specific items such as locations. Some common example categories might be things like "Articles, Trip Reports, Gear Reviews, Travels, Projects". Each category you enter should hold numerous blog galleries. If you find that your categories often contain only one matching result, perhaps you have too many and/or too specific categories and you should use fewer/broader categories.
Tags are well suited for more specific terms such as locations like "Colorado", "Grand Canyon", "Austria", etc.