body - Style for the body tag of your website.  Controls items such as the page background, etc.

div - Default format for all DIV (division) items in your template.  This is a catch-all that only applies when no other style has been defined for an item.

hr - Default format for all HR (horizontal line) items in your template.  This is a catch-all that only applies when no other style has been defined for an item.

p - Default format for all P (paragraph) items in your template.  This is a catch-all that only applies when no other style has been defined for an item.

td - Default format for TD (table data cells) items in your template.  This is a catch-all that only applies when no other style has been defined for an item.

tr - Default format for all TR (table row) items in you template.  This is a catch-all that only applies when no other style has been defined for an item.

ul - Default format for all UL (unordered list) items in your template.  This is a catch-all that only applies when no other style has been defined for an item.

a:link - Default hyperlink styling.  Applies when a link does not have a specific class defined.

a:visited - Default style for visited hyperlinks.  Applies when a link does not have a specific class defined.

a:hover - Default mouseover behavior for hyperlinks.  Applies when a link does not have a specific class defined.

* Note: Additional overall styles (such as H1, H2, etc) can be defined for Joomla! as well.