A Gallery 3 theme is the skin for the application. Where modules provide functionality, it’s up to the theme to figure out where the feature should be placed on the page and how it should be decorated with colors, fonts, borders, etc. The theme takes precedence over all modules and can override any controller, view, helper, library or model in the system.
There are two types of themes:
Any Gallery 3 install can have a site theme and an admin theme. They are designed separately because typically the admin interface has very different needs from the site interface, and it will have less users who are more savvy. As a result, there will probably be far less admin themes, and many more site themes. This relieves the average themer of the burden of writing a nice admin theme; they can focus on just creating a beautiful site theme.