It is the actual pages that the artists work on which are then used to generate the comics books that we all know and love. They are generally oversized white pages and can be either the cover or interior pages. If you're interested in purchasing original art (also commonly referred to as OA) , they can start as low as $20 a page up to and over $10,000 (or even $100,000) a page. Like anything collectible, prices in the OA market are on the rise and are really governed by what someone is willing to pay since they are unique original pieces of art.
Comics are often referred to as one of the arts invented here in America. We can let the scholars debate this but for our purposes we need to look at some of the history of comic book production to get a better understanding of the original art market. We will focus on original comic book art, but there are some applicable principles to original newspaper comic art and original animation cels.
Comics proved to be enormously popular in the 1930s, originally reprints of newspapers strips, and were soon supplemented with original strips. This is the genesis of the modern comic book industry as we know it. It is important to remember that its origins were as a disposable medium. To that end, they set up production houses with a premium on speed and efficiency so the assembly line method was adopted. Traditionally, writers would create a script, the script was penciled on oversized paper. The technology of the time would not pick up the penciled lines clearly enough so the pencil work was passed to an inker. The inker would then provide the finishing touches, darkening the art, filling in lines and providing shading. A good inker could embellish art to add character/texture, while a poor inker could obscure and even obliterate nuances in the pencil art. After inking, a letterer would add dialogue, narration, and sound effects. These were all applied to the same oversized page. Corrections would be made directly to the page. At that point, the page would be photographed/scanned to create a master. The coloring process would be applied afterword. The coloring process has changed over time but for our consideration the original art pages are not in color.
Often these beginning comic books would be uncredited, but that didn't last long. Most of this work was done as work for hire by contractors and not employees. There was no value seen in the pages except as a means to an end. Many of them were destroyed, given away, or lost. Eventually a market developed for these pages and they started to change hands at conventions in the 1960s. Eventually (after much work, legal action, and advocacy from the creative community) the practice became to return the original pages back to the artists. Upon return of the pages, they may be split among the members of the creative team. The pencil and ink artists definitely split the art with some pages being gifted to a writer or other member of the team on occasion. We are being a little vague about the exact history because the legal rights and status of creators has been and continues to be debated by historians and in the courthouse.
Sometimes partial advertising pages would be sold. In that instance the original art page would contain the artwork and blank space where the advertisement is finally placed.
Another great way to examine Original Art is by checking out the wonderful IDW Publishing Artist's Editions. The format has been copied by other publishers; Dark Horse, DC/Graphitti, Dynamite all put out versions of these large hardcover books that reproduce original art at original size with color scans of the black and white originals so readers can see every blue-line, white-out correction, margin note, etc. on the original art.
Lettering - Originally all lettering, captioning and sound effects were done by hand directly upon the original art page. Around the 1990s (depending on the publisher), digital techniques allowed the lettering to be applied after the pages were scanned. Originally it was printed out and manually applied to the original pages but eventually that practice was discontinued. Most modern pages will not have any lettering. Usually modern art has no word balloons or caption boxes but I have seen a few pages that do apply it. One writer/artist explained that as he was the one man behind the entire comic, those balloons saved the time of doing the artwork behind where they would lay, in the past the standard was to paste the word balloons right on top of the completed art:
Sample page with lettering: Jungle Action #9 page 16 - 1974 (pencils: Gil Kane inks: Klaus Janson letters: Tom Orzechowski) - This is a good example as you can see where a couple of the word balloons have come off, you can see the art underneath discolored from the glue that was used to affix the word balloons to the art.
Digital advances - Digital advances have greatly sped up communication among all people, this includes communications between the art teams. This includes you reading this now on the internet instead of in a printed magazine. The penciller will often send his work electronically to the inker who can print out the pencils in blue (as the blue is not read when scanned for the master). The inker will ink over the blue lined copy. That is scanned and sent to the publisher. Accordingly in this instance you have two "original pages" the page worked on by the penciller and the page worked by the inker. Be very careful when buying inked art from the 2000s and onward. Typically these "inks over blue line" originals can be had cheaper, you should be careful not to pay the premium price associated with a page with inks applied directly over the penciled pages. What digital advance hold for the future is anyone's guess.
To ink or not to ink? Sometimes no inker is used and the work is done directly from the pencil stage. Improvements in technology mean that if the pencils were completed with sufficient detail the master could be generated directly from those pencil drawings. Alternatively, it is also common for one artist to provide the finished work (absent color). Since 2008 the Inkwell Awards have been recognizing this dying art form and acknowledging inkers from the rich history of comics in the Joe Sinnott Hall of Fame. The use of an inker is certainly on the decline, Bob Bretall has been on the Inkwell nomination committee since 2012 and has certainly noticed that the field of Ink Artists has been growing smaller each year.
Gene Colan (art produced directly from pencils) from Harlan Ellison's Dream Corridor v2, "Moonlighting" page #11 - 2007
The Demise of Original Art? Some artists now choose to work entirely in digital format which means there is no original page at all. some do it because it's quicker or because the digital tools allow them to do things they cannot (easily or at all) draw using physical media. Some of these artists offer to create a high quality print of their art that they may (or may not) guarantee to be the only one they issue, making it a "1 of 1" print that is an analog of original art, but it's really not the same thing.
These changes represent the vast majority of industry practices from the major publishers but there are still a few independent creators that still use hand lettering or other traditional methods. Given the efficiency gains in these modern methods, it is unlikely that the old ways will ever become the majority again.