About original comic art:

Original art pages and covers are one of a kind items.
For any page of a comic book, there is only one page actually drawn by the artist.

Types of original art may include:

Bristol board with pencil and inks:
The artist's pencils are usually done in non-reproducing blue with black ink over the pencils. Sometimes, corrections (correction-fluid may be seen, as well as production notations in the page margins. Up until the last 10 years or so, the bristol boards contained word balloons and lettering. This work is usually now done by computer and newer original art pieces have only the artwork.

Production proofs:
These are often paste-ups of photocopies used in the final camera-ready copy. There is usually no actual drawing on these pages. These pieces are worth considerably less valuable than the actual original art, but desirable pieces can still go for high prices.

Color proofs: 
Color proofs are production proofs used by the colorist to work on a color scheme.

Artist sketches and commissions:
Comic book artists often appear at conventions or in-store signings and do sketches for fans, or accept commissions to do pinups. These are usually not published art, but are often still referred to as original art.