Viewfinders are optical systems that present a view very similar to what the lens actually sees. By contrast, the LCD panels on most cameras let you see what the cameras lens sees. This can be useful in a number of situations, especially for close-up photography. The Nikon 990 tilts the LCD from the lens so that you can even hold the camera above your head and look up at the LCD while taking a picture, which is great for crowd shots. Unfortunately, LCDs are often difficult to see in bright sunlight.
Most digital cameras use flash memory cards to store the images that are taken.
You then transfer the images to a computer, erase them from the card, and take more pictures. They are essentially identical in functionality, though they are incompatible with one another. Physically, the Compact Flash is almost square, and a little thicker than the other cards. There are some technical and performance differences between the three types of media, but in practice these differences are relatively unimportant. So far, CompactFlash has tended to be available in slightly larger sizes than SmartMedia and Memory Stick.