The dropdown allows you to merge a button with an icon (down arrow), and a list of links.
For more informations on the icons used, consult the dedicated page.
Take a look at the button page to get all the necessary information.
The dropdown is largely based on the
.ms-btn classe, so it is possible to use the styles applied to this class, this can be used to change the button colors.
It is also possible to change the dropdown size.