Short answers, pulled from the story.
Traditional animation, also called cel animation or hand-drawn animation, is a technique in which each frame of a film is drawn by hand. Animators draw sequences of images on transparent plastic sheets called cels, which are layered over painted backgrounds and photographed in stop motion, one frame at a time.
Earl Hurd and John Bray invented the cel animation process in 1914. The cel allowed different elements of a scene, such as a character and a background, to be drawn on separate transparent layers rather than redrawn together on a single sheet for every frame.
The last major feature film to use traditional ink and paint was Satoshi Kon's Millennium Actress, released in 2001. Bill Plympton's Hair High (2004) was the last animated film to use traditional cels.
Walt Disney Animation Studios began using digital ink and paint in 1989, first applying it to the final rainbow shot in The Little Mermaid. The Rescuers Down Under, released in 1990, was the first major feature film to use digital ink and paint entirely.
The multiplane camera was used to create a sense of depth and realistic parallax in two-dimensional animated films. Artwork was painted on separate glass planes at different distances from the camera; when the camera moved, nearer planes shifted while distant elements like the moon or sky stayed put, mimicking the way depth appears in real life.
Rotoscoping is a traditional animation technique invented by Max Fleischer in 1915, in which animators trace over live-action film footage frame by frame to produce lifelike movement. It was used in films including Snow White and the Seven Dwarfs, Sleeping Beauty, and Peter Pan, and in music videos such as A-ha's "Take On Me".