A digital camera viewfinder is the part of the camera that is used to frame and setup a photograph. It is usually located on the back of the camera, and can be either an optical viewfinder or a digital, or electronic viewfinder (EVF).
There's also another type of viewfinder, though it is often lumped into the digital viewfinder category: the viewfinder screen. This is the screen on the back of most DSLR cameras where photographers can change settings, scroll through captured images, and in some cases, make some minor changes or corrections to the image. This screen, which is usually about two to two and a half inches square, can also be used to frame a scene and focus the camera.
And, in some cases, the viewfinder screen is a better option than the optical or digital viewfinder located on the body of the camera. For example, if you're shooting in a place where it's awkward to hold the camera to your face, the viewfinder screen may help you focus better, especially if it's an articulating screen that can move left and right as well as up and down.