also here, big kudos to the TTD folks for this info:
Each of these three terms represent different employments of the md5 checksum algorithm. It has the ability to reduce any file to a single 128-bit number which is unique to that file. Most audio and video trading websites require the generation of checksums for any files as it has the ability to compare two files and determine if they are identical. If your file matches the checksum of the original file, you can confirm that it has not been altered in any way (intentionally or unintentionally) or corrupted during download. Verifying checksums every time a file is transferred is an essential step in the trading of lossless music.
For the more mathematically adept users, here's an actual description of RFC 1321 - The MD5 Message-Digest Algorithm.
If you were brave enough to muddle through the second link posted above, you will have discovered that the output from running an md5 check is a single 128-bit number. To make it easier to read, many programs that run the md5 algorithm will actually output an equivalent 32-digit hexadecimal number which has the same binary result as the original 128-bit would have. This makes reading it simpler using only 16 alpha-numeric characters (0-9, a-f). The result will look something like this (this is an shntool md5, .st5, which is what is desired at TTD):
cb8a42007fe797435826a76cb527be01 [shntool] sample2007-04-15t01.flac
25eb2520a33ed2b88edf41f5d6f09505 [shntool] sample2007-04-15t02.flac
fbfced772c4d478f57072ffe9aeb2a69 [shntool] sample2007-04-15t03.flac
Each line contains the 32-digit hex-coded checksum, followed by the filename of the file it was run on. This 32-digit checksum is what most computer users are likely to see as the end result of a checksum calculation.