The dock is a row of icons centered on the bottom of the screen.
Minimized windows become icons in the dock.
The dock takes advantage of Quartz's ability to apply vector transformations to bitmapped images. First, all the icons in the dock are dynamically scalable. Move the icon-size slider across its range of motion and you'll see the dock icons dynamically scale all the way down to a tiny 16x16 pixels. The results are similar to what you'd get if you imported the 32-bit 128x128 icon bitmaps into Photoshop and scaled them. As you can see, the 128x128 photographic icon scales much more nicely than the traditional 32x32 line art icon.