what is an "av" file?

Hi. Can anybody explain to me what an AV (not "AVI") file is? When I capture raw footage, using either batch capture or capture now, Final Cut Pro 5.0.4 names each of the incoming media clips/files with the name I gave it in the capture window. But when I capture something from tape that's not raw footage -- that's been edited before by someone else -- I get one file with the file name I gave FCP in the capture window, and then a whole bunch of ancillary files named with the "root" file name plus the following designations: -av, -av1, -av2, -av3, etc.

What are these files? I can't find any information about them in the FCP documentation. I made the big, big, big mistake of trying to rename the folder these files are in (I now know how horrible this is to do), but I'm wondering how much of a factor, if any, these "av" files are in the problems that arise when FCP tries to identify what to use when.

Thanks!

Rumi Girl