Registered members have a choice of the Vertical menu on the left side of the screen along the top. The new user default is to see the Vertical menu on the left. This can be changed for registered and logged-in users by going to the Members -> Prefs -> Site Style option and selecting your desired Navigation style. (Page here: »/prefs)
The default Vertical Menu Style:
The Horizontal Tabbed Style:
The selected style will be used throughout the site.
(Note: The images are not quite current, but the basic info is the same.)