The viewfinder is the optical window you look through to compose the image.
The simplest and most common type in use. It is not very accurate in framing the scene as it doesn't look straight through the lens and is mostly slightly offset both horizontally and vertically. It only shows about 80% to 90% of the actual image.
Shows the image on a very small LCD screen. It is used in the same way as the optical viewfinder but is much more accurate since it doesn't suffer from parallax error.
TTL optical viewfinders
Usually only found on high-end digital SLR cameras because of its complexity and expense. The most accurate system of all since you can look at the scene straight
through the lens through a prism or mirror system.
Featured on almost every digicam nowadays. It lets you review images, change camera settings or frame the scene. Since it is much more accurate than optical viewfinders, many users compose the image through the LCD screen instead. However, they drain batteries fairly quickly and they may be difficult to judge in bright sunlight.