Check md5 checksum
 First of all, what is md5 ?
MD5 is an algorithm that is used to verify data integrity through the creation of a 128-bit message digest from data input (which may be a message of any length) that is claimed to be as unique to that specific data as a fingerprint is to the specific individual. MD5, which was developed by Professor Ronald L. Rivest of MIT, is intended for use with digital signature applications, which require that large files must be compressed by a secure method before being encrypted with a secret key, under a public key cryptosystem. MD5 is currently a standard, Internet Engineering Task Force (IETF) Request for Comments (RFC) 1321.
Okay this definition is extracted from http://searchsecurity.techtarget.com
Simple Usage, we use md5 to verify the correctness of something we have downloaded. You always provided with md5 fingerprint while downloading large file such as iso images for linux distro. It is just a line of 128 bit string.
 Why I need to do the checking? Is it important?
To ensure the file you downloaded is not corrupted. Yes, it is important. If you try to burn the iso that is corrupted to CD, the CD may not work properly.
 How to check?
Uses md5sum, the simple way is run the command line straight away. Let say you want to check the file ubuntu-6.10-desktop-i386.iso.
With this, it prints out the 128 bit fingerprint strings. Tally the string you obtained with the provided one. Provider do the same way to obtain this string and publish to the site.
Another way let say you have more files to verify, you can create a text file, such as md5sum.txt
283158c7da8c0ada74502794fa8745eb ubuntu-6.10-alternate-amd64.iso 549ef19097b10ac9237c08f6dc6084c6 ubuntu-6.10-alternate-i386.iso 5717dd795bfd74edc2e9e81d37394349 ubuntu-6.10-alternate-powerpc.iso 99c3a849f6e9a0d143f057433c7f4d84 ubuntu-6.10-desktop-amd64.iso b950a4d7cf3151e5f213843e2ad77fe3 ubuntu-6.10-desktop-i386.iso a3494ff33a3e5db83669df5268850a01 ubuntu-6.10-desktop-powerpc.iso 2f44a48a9f5b4f1dff36b63fc2115f40 ubuntu-6.10-server-amd64.iso cd6c09ff8f9c72a19d0c3dced4b31b3a ubuntu-6.10-server-i386.iso 6f165f915c356264ecf56232c2abb7b5 ubuntu-6.10-server-powerpc.iso 4971edddbfc667e0effbc0f6b4f7e7e0 ubuntu-6.10-server-sparc.iso
First column is the md5 string and second column is the location of the file. To check all them from file, do this:
md5sum -c md5sum.txt
The output will be like this if success
... ubuntu-6.10-desktop-amd64.iso: OK ubuntu-6.10-desktop-i386.iso: OK ...