In the style.css file in the first comment block, there is a optional "Tags:" directive. What are these tags used for? When are they used?
The theme tags are used to sort/categorize themes in the WordPress theme directory according to features or characteristics.
For example, if your theme supports two columns (a typical blog format), then you use the
These tags are only used on the WordPress theme site and are completely optional. However, it means that people using those search filters might not find your theme and that's no good. If you're making a custom theme for yourself or for a client and you're not going to upload it to the WordPress theme directory, you don't need to include them.
For a full list of tags you can use, check http://wordpress.org/extend/themes/about/.