Tab order is generally in number format and it is quite important to consider this when building a form with lots of controls. This tab order is part of the safety net that the user does not even realise is there. You may want a user to go through a form in a certain way so they do not miss information or processes that are needed to be carried out such as username then password in that order.
It's quite simple really, all you need to do to set tab order is click on each control in turn and set the tab order to reflect the tab movement that you want. The property is listed as tabindex and most controls will have a tab order. Anything that the user has to interact with will have a tab order property.
If there are controls that you do not want to be included in the tab order than just make the value much higher than the set of controls you have in the tab order. for instance you may want to exclude a critical button from a tab order, this is because a user can inadvertantly set focus and hit the return button, which will then activate a process you only wanted to be activated with the mouse.
The tab index will be set in the sequence of the order of the control that you put on the form, you may want to change this as you do your development work however.
Generally speaking many people do not use tabbing and use the mouse to click in fields and on buttons, however commerical users and people who work with computers daily will generally be more accustomed to tabbing and it has to be built into any computer programme to handle this type of usage.