md5sum (Checksum of a file)
definition of md5sum:
The program md5sum will tell you the md5 checksum (md5sum) of any stream of data or a file. It is a 32-digit hexidecimal number (with the a-f characters lowercase). If the md5sums of two files are identical, it is likely that the files are also identical. If the md5sum is different, the files are definitely different. This property is used to verify the integrity of downloaded packages.

To get the checksum of a file in linux using md5sum,
# md5sum file.tgz

You can also do md5sum to multiple files by enumerating the files or by the use of wildcards in linux
# md5sum file1.tgz file2.tgz file3.tgz
or
# md5sum file*.tgz

You can also redirect them into a text file
# md5sum file*.tgz > md5sum.txt

To verify on the other end, you can run the md5sum and compare the result
# md5sum file1.tgz

Or, if you want to verify all files that were saved into md5sum.txt
# md5sum -c md5sum.txt


For more info, http://en.wikipedia.org/wiki/Md5sum