kmosx2: What is that transparent bar?

It is called the "Dock." Some of its features may remind you of Microsoft Windows taskbar, but it acts quite differently than the taskbar. When the machine first starts in X whether after an installation of X or just starting X for the first time on a machine that has X installed, the Dock will appear at the bottom of the screen. It may be moved to the right or left side of the screen without any third party addons. Unlike Windows you'll notice:

1. It has the trashcan. The trashcan retains items in the order they were stored on the hard drive when you drag them over to the trash, and any folder hierarchy they were before. They can be dragged back, or if you did nothing with the computer since you dragged it to the trash, a simple command key (apple logo and cloverleaf logo on the keyboard)-Z or selecting Undo from the Edit menu in the Finder will move them back.

2. Clicking once on any item quickly and letting go will open it.

3. Click and hold down on any item will bring up a little menu next to it of things you can do with it.

4. The dock is split into two sections with a thin divider between the icons. When the dock is on the bottom of the screen the icons on the left are all the application shortcuts you choose to be there, and any that were left there by default. Simply dragging an icon off that area of the dock and letting go will make the shortcut disappear. But it DOES NOT DELETE THE FILE FROM THE COMPUTER!
You can always drag the file from the computer back onto the dock to have access to it again. Applications will have black triangles under them if they are open, and once open the menu from holding down the mouse button on it will let you keep that item in the Dock after quitting it. A misbehaving program may be force quit by holding down the Option key when using that menu.

5. The right section of the dock will store any folders or documents you choose to drag there including hard disks and other media that store data. Also any window in the computer can be minimized to go to the dock by clicking on the middle button in the upper left corner of the window. If you haven't changed the color scheme, it is yellow. whether you have or haven't mousing over that minimize button shows a - sign. Clicking on that window's icon in the dock again maximizes it. Folders and media disks will show in their click and hold menu a hierarchical menu of the contents of that folder or media for easy access to its contents. Note again, these are all only shortcuts and dragging any item off the dock that is a folder or media does not remove it from the system.

6. The divider between the folder/window and application area has a click and hold menu for changing characteristics of the dock like its minimizing behavior, hiding behavior, and position of the dock on the screen and a quicker click on the divider will let you grow or shrink the size of the icons and the dock itself.