Well, here goes.

Cels, as some people are aware were used in animation before CG really started catching on.
Think of cartoons you've seen, any cartoons. Cartoon commercials, cartoon tv shows, cartoon movies.
The Last Unicorn, or maybe An American Tale. Maybe you've seen Heavy Metal, or Rock and Rule.
Cartoons were never just kids' stuff. There are a million genres from Bambi to Aeon Flux.
Up until just past the turn of the millenium, Cel animation was the way things were done. Meticulous
line tracing of multiple shots of the same character in just slightly different poses. Painted on acitate and photographed in order, the resulting film fools the eye by making the rapidly changing images seem to move.

Essentially, like any other collector, a cel collector has their favorites. In this case, beloved animated features, pivitol scenes or favorite characters. In many cases, even though it may have taken 10-25 cels to dreate the scene, some cels in the scene are more prized, as they are of better quality. The first cel in a sequence (labeled A1) will set the inital positions, and is usually drawn and/or painted by a higher qualified cel artist than subsecuent cels in the set (A5, or A14). End cels, (cels
that mark the end of that particular sequence or "cut") are maked as, for example A5E or A5End. These are also often of higher artistic quality than the cels in between the begining and end cels.  (The cels in between often are drawn/colored by "in-betweeners", a term meaning anything from "less-talented artist" to  "random mediocre guy in an assembly-line type set-up" to "no-talent-hack-janitors-that-got-roped-into-painting-in-an-emergency")  There are also often multiple layers of cels, (A1, B1, C1) each depicting a different charater moving in a different way. These are sometimes found separated from their other layers.

High quality cels of favorite characters from important scenes in popular animations have been known to sel for THOUSANDS of dollars. (Yes, THOUSANDS! For a piece of clear acitate with some paint on it!) Sadly, I do not own any of those. XD  I own mostly cels from animations that no one else seems to have heard of.

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Now for  Lesson two:  What are genga, douga, etc??

If you've done any searching on the internet, or just on eBay, you've probably
wondered at a few terms.  Often, you see  "Sketch, not CEL" or the words "Genga" and "Douga"
in the descriptions. Upon viewing, you see these are indeed, not cels. They are pencil outline drawings
sometimes with notations in pencil or pen, and sometimes very roughly drawn.

In the process of making an animated feature, there first comes a script, then a storyboard.
A storyboard is a small bunch of sketches that show what you want to happen in a scene. There are copius notes about placement of character, what they are doing, where they are going.  There may also be "settei" or detailed sketches of the characters, full body, and close up features, as well as multiple expressions and poses. There are settei for layout of the backgrounds, important items, and special costumes as well.

Next is a "Genga". This is a larger, rough, but more rdetailed sketch than the story board. There is a seperate sketch for each cel that will be filmed, and the genga is sort of a rough draft. The genga shows where the placement of the sketch will be on the background, as well as movement in the scene.

Then comes the sketch or "Douga". This is what a cel is based on. These lines are traced onto the acitate with either hand inking (for very early animations)  Xerography or the ATP process.  Basically, it's a map. A tracing guide. It's the last step before acitate.  In the newer CG animated features, this is also the last step before computer graphic coloring.

Many times you will see genga or douga for sale from newer seires, and many people collect these, since there are no production cels. They sometimes have non-photo blue drawn rough sketches on them before the final drawing.

Sometimes a studio will release "post-production" cels or "Rilezu" cels. These are commercially made cels that are made in lots of 250 or 500, or often more. They are all the same, and are usually hand painted from the genga sketch used to make the CG anime. They  are usually nicely matted, and come with a "Certificate of Authenticity".   They were never used in the making of the cartoon. There are differeing opinions on whether these count as "real" cels.  Let's not go into that here tho. =D

Then there are always "Fan cels" or "Circle cels". These are cels that are drawn and painted by fans of a series, not
the studio. They are sometimes very nice, and can be mistaken for the real thing, so be sure to look for "this is a fan-cel" in the description.  When in doubt, ASK!