By default, you have one type of forum: regular. Suppose you wish to create another group called 'special'.
First, make sure you've switched forum types on at Admin Panel -> Settings -> Switches. Then go to Admin Panel -> Forums -> Forum Settings. 'Forum Types' is near the top. Add a comma after the 'regular' you see there, and type your new group (do not leave a space). In this case, we'll now have 'regular,special'. Also choose the friendly english display language for it which will be used on the edit/submit forum pages. Save your settings.
Next, edit your 'main index' (main.tpl) and 'display articles in forum' (displaylinks.tpl) templates at Admin Panel -> Themes -> Manage Templates. In order to create a unique style for this forum type, copy the style which has been used for regular forums and change it to fit what you wish for your new group. For example:
<!-- BEGIN SPECIAL -->
<blink><b>Extra special type of forum: <a href="{CATURL}">{CATNAME}</a></b></blink> <br >
<!-- END SPECIAL -->
<blink><b>Extra special type of forum: <a href="{CATURL}">{CATNAME}</a></b></blink> <br >
<!-- END SPECIAL -->
You must add the new type code for each forum type you add, or forums of that type will be invisible.
By placing that HTML where you want it, you can make the special forums appear in a special place such as at the very top.
Forum types apply to both top level forums and subforums, but not within the <!-- BEGIN SUBSUB --> nested subforum list inside each forum display box. You could leave that list unseparated, remove the <!-- BEGIN SUBSUB --> area or you could use conditionals along the lines of <IF {CATTYPE} is special>{CATNAME}</IF> in there. If you do the later, you'll need to add type to the subhtmlfields tweak so that the type field gets included in the cached data.