User styles are categorized by what they change.
Site styles make changes to particular web sites. They will generally do things specific to the web site they apply to, like tweak the display, remove sections, or even completely redesign them. There are 79,605 site styles.
App styles affect the interface of Firefox, Thunderbird, and other Mozilla programs. They can change toolbars, menus, dialogs, tabs, extensions, and any other interface item. There are 5,363 app styles.
Global styles make changes to all web sites. They will make general changes like adjusting the font or colors. There are 7,733 global styles.
If you're feeling adventurous, you can also check out the style graveyard, where broken styles go when they die.