Disney has almost been synonymous with the term animation since the early 20th century, and they've produced some of the most well-known and beloved works in cinema history. Beyond their fantastic ability to tell stories that resonate with people of all different ages, the technical proficiency of Disney's hand-drawn animations was a step above most of the industry. Part of that was due to the invention of the Multiplane Camera, which literally gave 2D animations a third dimension, and in the process, brought them to life.

Here is Walt Disney himself in 1957 to explain the camera:

While this wasn't the first multiplane camera, it allowed for far more complex movements (from Wikipedia):

The most famous multiplane camera was invented by William Garity for the Walt Disney Studios to be used in the production of Snow White and the Seven Dwarfs.[3] The camera was completed in early 1937 and tested in a Silly Symphony called The Old Mill, which won the 1937 Academy Award for Animated Short Film.[4] Disney's multiplane camera, which used up to seven layers of artwork (painted in oils on glass) shot under a vertical and moveable camera,[3] allowed for more sophisticated uses than the Iwerks or Fleischer versions, and was used prominently in Disney films such as PinocchioFantasiaBambi, and Peter Pan.

You can see more examples of the multiplane camera in use in Disney productions in this video (which is only available to watch on YouTube):

This camera became obsolete as computer-assisted animation and computer-generated animation became more prevalent, and the last Disney film to use the technique in any significant way was The Little Mermaid. The natural progression of these ideas is actually replicating what a real camera can do in the virtual space, as seen with a film like Pixar's Wall-E and its use of anamorphic "lenses" to give a distinct feel to the images. The opening of the film actually feels like an homage to those earlier Disney films, before moving into the anamorphic look (as a side note, cinematographer Roger Deakins was a visual consultant on the project):

For more on the Multiplane Camera, check out the link below.

Link: Multiplane Camera -- Wikipedia