Well, an 80A is a "correction" filter. Back in the "old days" of film, film was balanced to different lighting. You had "indoor film" which was balanced for tungsten lighting (aka regular light bulbs), "outdoor film" which was daylight balanced, etc.. Now the problem was, what if your camera was loaded with indoor balanced film and you wanted to shoot something outdoors? You could stop and change the film in the camera (and potentially loose the rest of that role of film) or you could slap on a color correction filter...in this specific case an 85B. The 80A was just the opposite...it was so you could use daylight balanced film indoors.
Since I don't know your level of experience here, the thing you have to understand when it comes to light is that it has different color or "temperatures". We humans have this lovely built in device that we call a "brain" that compensates for this automatically. If you take a piece of white copy paper for example and look at it in bright sunlight, it looks "white" but if you take it indoors and look at it under a lightbulb (or floresent), it still looks "white". The problem is, the camera, or more specifically film or image sensors, don't see things that way...they see light in it's actual color or temperature. Under a regular lightbulb, the white piece of copy paper is actually a little orange and under floresents, it a little blue. Our eyes and brain adjust and compensate, but a camera does not (well, barring the auto settings on a digital! LOL).
Now if you are indeed shooting film, you'll find that having an 80A filter is wonderful...it will allow you to use a daylight balanced film with indoor tungsten lighting. If you're using a digital camera however...well...it's not really that big of a deal in itself. With most digitals, you can adjust for the lighting conditions right on the camera and to be honest, the auto settings do a hell of a job in 80% of your shooting situations.
With that all said, I would like to add though that even if you are shooting digital, you can still use that filter very effectively for "effects". Don't be afraid to experiment with it...try shooting some water or something with it and you can get some pretty interesting photos! LOL! I've actually used an 85B on sunset photos to really punch up the colors of a sunset (saves me the trouble of doing it later in Photoshop! LOL!!!).
As far as websites, as Pat said, just try rewording your search in Google. There's a TON of info out there. I just typed in "80A Filter" and got a bunch of results that had nothing to do with Ebay, including Pat's Wiki reference.
Alrighty, I hope this helps!