This type of art is created and used during the pre-production phase of a project; often when the production committee is deciding whether to give it the green light. This may include: proposals, cels, sketches, and painted art. Disney has been known to use sculpture as well. The medium(s) used will depend on the artists' preference and the requirements of the committee. Concept art can be much more difficult to find than other types of art created during the animation process.
See some examples of concept art below.