Only the default tabs can be exclusively set in the Tabs dialog box. Left, Right, Center, Decimal and Bar tabs can be set through the ruler. You can't set leaders for tabs there though, but they are not kinds of tabs.
All types of tabs can be set using the ruler. You can change the tab type first, by clicking on the button beside the ruler. You can then click on the ruler to set the tab. What you can't do on the ruler is define leaders for the tabs. That may be what you are referring to. You can do that through the tabs dialog box. You could also be referring to default tabs, but they have already been set. You can change their measurements in tabs dialog box, but not on the ruler itself.
left
click the launcher in the paragraph group,then click tabs
The ribbon is the answer you are looking for, but some dialog boxes also have tabs on them.
You could get tabs if you are using I.E.7, Firefox, and any other browsers that supports tabs. You might be able to get tabs in add-ons.
Click little arrow that shows to bottom in Paragraph section at Home tab and look for tabs at bottom of paragraph dialog which will open.
Group Membership
set
number of columns and number of rows.
You are supposed to use a tab as an indention, and indentions come at the beginning of each paragraph or dialogue.
You can remove the tabs in page using hook_menu_alter.
One site is 911 tabs. Also, you could use MX tabs.