A viewfinder is basically the small lens you look through on the camera to see the image to see if it's in focus and aligned to your specifications. The magnification of what you see will change with different focal lengths of your lens. A viewfinder is also known as a finder or projected screen. You can also get interchangeable variable viewfinders if you prefer one over your cameras viewfinder.
On some cameras the viewfinder isn't always exactly in the same place as the lens so it can show a slightly different image to what will appear on film. This is called parallax error. Inside the viewfinder is the focusing screen, which can usually be changed for different situations. You can also get magnifiers for your viewfinder to enlarge the image to help you focus. Make sure you are comfortable with the camera's viewfinder before buying it. You can't photograph what you can't see. Variable viewfinders generally cost a few hundred dollars.