The waist-level finder is a type of viewfinder

Viewfinder

In photography, a viewfinder is what the photographer looks through to compose, and in many cases to focus, the picture. Most viewfinders are separate, and suffer parallax, while the single-lens reflex camera lets the viewfinder use the main optical system. Viewfinders are used in many cameras of...

 that can be used on twin lens

Twin-lens reflex camera

A twin-lens reflex camera is a type of camera with two objective lenses of the same focal length. One of the lenses is the photographic objective or "taking lens" , while the other is used for the viewfinder system, which is usually viewed from above at waist level...

 and single lens

Single-lens reflex camera

A single-lens reflex camera is a camera that typically uses a semi-automatic moving mirror system that permits the photographer to see exactly what will be captured by the film or digital imaging system, as opposed to pre-SLR cameras where the view through the viewfinder could be significantly...

 reflex cameras. While it is typically found on older medium format cameras, some newer and/or 35 mm

135 film

The term 135 was introduced by Kodak in 1934 as a designation for cartridge film wide, specifically for still photography. It quickly grew in popularity, surpassing 120 film by the late 1960s to become the most popular photographic film format...

 cameras have this type of finder (perhaps as an option).

In the reflex camera, the light from the lens is projected onto a focusing screen

Focusing screen

A focusing screen is a flat translucent material, usually ground glass, found in a system camera that allows the user of the camera to preview the framed image in a viewfinder. Often, focusing screens are available in variants with different etched markings for various purposes...

. The waist-level finder makes this screen viewable from above, where the image is seen upright but reversed left-to-right.

The eye-level finder is an evolution of the waist-level finder, using a roof pentaprism

Pentaprism

A pentaprism is a five-sided reflecting prism used to deviate a beam of light by 90°. The beam reflects inside the prism twice, allowing the transmission of an image through a right angle without inverting it as an ordinary right-angle prism or mirror would.The reflections inside the prism are not...

 or pentamirror

Pentamirror

A pentamirror is an optical device used in the viewfinder systems of various single-lens reflex cameras instead of the pentaprism. It is used to reverse again the upside-down and laterally reversed image coming from the reflex mirror....

 to correct the image while making it viewable through an eyepiece at the rear of the camera.

Advantages

• The photographer is looking down into the camera, attracting less attention compared to an eye-level finder. This makes it better for street
  Street photography
  Street photography is a type of documentary photography that features subjects in candid situations within public places such as streets, parks, beaches, malls, political conventions and other settings....
  
   or candid photography
  Candid photography
  Candid photography is photography that focuses on spontaneity rather than technique, on the immersion of a camera within events rather than focusing on setting up a staged situation or on preparing a lengthy camera setup.-Description:...
  
  .
• Even when they know they are being photographed, the subject may feel less "targeted" because the photographer is not looking at them.
• The camera is often held against the belly or resting on something (e.g table, the lap, etc), resulting in less motion blur
  Motion blur
  Motion blur is the apparent streaking of rapidly moving objects in a still image or a sequence of images such as a movie or animation. It results when the image being recorded changes during the recording of a single frame, either due to rapid movement or long exposure.- Photography :When a camera...
  
  .

Disadvantages

• The image is reversed left-to-right, making it tricky to follow moving subjects.