CRC is the string of letters and numbers you may have noticed on the topic of fansub groups as they release stuff (e.g. "E6AB3A08"). This can be used to check whether the file is corrupt or not. If you have downloaded a file, and its size corresponds to the original, but it doesn't work, the reason could be corruption. Of course, in case of anime divx, the reason might be wrong codec also, but checking the crc never hurts.
There are a few ways to check crc. First one is to download a program that checks it by using an sfv file (betcha you've been wondering what those are). You can get one from IsoNews . Download the file, unpack and read the documentation on how to use. It's very simple program, so I'm sure you'll handle it. :p (you can always wish ;)
Oh, and just in case... For windows users, the actual program is in WIN32 subdirectory. I'm sure *nix users can fin theirs without problems :)
Other way (assuming you're using mirc) is to read the crc with mircs internal checker. This can be done with the command "//echo $crc([drive]:\[path]\[filename]")". Without quotes of course. I.e. for me, it would be something like "//echo $crc("g:\divx\animefolder\animeclip.avi")" Substitute with your own drive, path, and filename as necessary (don't forget file extension).
If the crc isn't mentioned anywhere, you can also open an sfv file in a text editor and
check it there. Of course, if you're using pdsfv or some similar program, you don't need to. :p
Note: If you make an sfv of your file, and open it in text editor, you can compare that crc to the original.
If you use the mIRC command, compare the result and original crc to each other. If they're the same, the problem is somewhere else. If they're different, you can use Zidrav patch to repair the file (see zidrav section). This goes for sfv checker also. If it says the file is good, problem is elsewhere, if it says the file is bad, ask for a patch.