One of the first things that you'll notice on the Ubuntu desktop is that there are two taskbars, which are called “panels.” The one at the top contains your Start Menu on the far left side. It is broken up into three separate, but logical menus. The first is the Applications menu, which contains shortcuts to the applications installed on the system (organized by category) and the Add/Remove wizard. This is like the classic Windows Start Menu or the All Programs section in Windows XP and Windows Vista.
The middle menu is the Places menu, which contains shortcuts to your Home folder and the most common folders within it. It also has your attached devices, like optical, floppy, and storage drives, as well as shortcuts to network destinations. The Places menu has the equivalent destinations of the My Computer, My Documents, and My Network Places icons and Start Menu items in Windows. Also in the Places menu is your recently-viewed documents and desktop-search tool.
The third menu on the upper panel is called the System menu. It contains Administration and Preferences as well as the About and Help buttons. The Administration and Preferences sub-menus offer the functionality of the Control Panel in Windows.
After the System menu are shortcuts to Mozilla Firefox, the Evolution email client, and the Ubuntu Help Center. This section is no different than Windows Quick Launch and you can right click to add or remove these shortcuts as you see fit.
Along the right hand side of the upper panel are the network status, volume control, calendar/time, and user/shutdown applets. The volume, calendar, and user/shutdown buttons will expand with more functions when pressed, just like Windows. This is also where you'll get notifications to update or restart as well as device-status information. Some programs, such as your Bittorent client, instant messaging, and music player, will minimize to this location. This is essentially the Windows equivalent of Task Manager and Notification Area located in the lower right hand side of the Windows taskbar.
The bottom panel contains a "show desktop" button on the far left, which will minimize all windows when pressed. There is a list of all open windows in the middle of the bottom panel, just like in Windows. A workspace switcher (there are two desktops by default) and Trash icon (Recycle Bin in Windows) are on the far right side of the bottom panel.
Note: this is a guide for the default GUI included in Ubuntu. Any and all of the GUI elements can be moved, resized, changed, or removed, including replacing the entire GUI with another or with the command-line interface (CLI). The combinations of panels, widgets, gadgets, applets, icons, themes, and effects are innumerable.