In the past, animated films and series were painted and inked on transparent celluloid films. This is why people often refer to traditional drawn animation as cel animation.
Today, animated films and series are processed in computer software. It is faster, cheaper and cleaner. In the past, animation levels drawn on different layers had to be coloured with slightly different colours based on their position. This was done to compensate for the lack or over exposition to the light table lighting the animation stand.
Computer software also allow studios to ink and paint digitally. It is more economic and makes the correction of errors easier. In the past, systems had to be organized to move painted cels and allow them to dry. Smudge and error meant made that system very costly.