install a fresh wordpress setup on your local machine and then have a look at wp_usersmeta table . It contains meta(additional) information for each user.You (admin) can also make data entry specific to any user by this function update_user_meta() in usermeta table. and retrive the same using get_user_meta() funciton. 

All fields for all users are not necessary. meta keys can be user specific also. 

defaults meta key entries for admin in wordpress 3.6 -- 

first_name
last_name 
nickname 
description 
rich_editing 
comment_shortcuts
admin_color
use_ssl 
show_admin_bar_front
wp_capabilities
wp_user_level 
dismissed_wp_pointers
show_welcome_panel
wp_dashboard_quick_press_last_post_id
managenav-menuscolumnshidden 
metaboxhidden_nav-menus 
nav_menu_recently_edited
closedpostboxes_page 
metaboxhidden_page 
metaboxhidden_page  
wp_user-settings wp_user-settings-time


users and usermeta table
Whenever i try and close Skype on Windows, whether through the X on the top bar, or by right-clicking on the taskbar, it just minimizes it, instead of closing it.

How are you supposed to just quit??
