I always see this when i find a software.But i don't need it when i install the software.Then, what is the use of MD5 code?

MD5sums are used to verify a successfully downloaded iso.You run the command:
md5sum /the/directory/containing/iso
If the sums match your download was good.You should always do the md5sums before creating a cd from the iso image.
HTH
lynch

MD5 sums are provided on some sites to make sure that the file you have downloaded from the site is not corrupt.

In a nutshell:
The md5sum is a 128 bit checksum number that will verify the integrity of a downloaded file. If you run the md5sum program it will generate a number based on the downloaded file. If it matches the number in the md5sum file than you can be assured that the file has not been corrupted when downloaded.

I've done a quick search on checksums and have posted a few links.

http://www.math.utah.edu/~beebe/soft...dr/node15.html
http://searchsecurity.techtarget.com...211779,00.html
http://www.flounder.com/checksum.htm

I'm brand new to Linux but I gather that md5 is used for more than just checksums. I don't know how yet, but I think it is useful for password encryption and the like.

Michaelk successfully put md5sum in a nutshell for verifying file integrity.

MD5 is not an encryption mechanism. It produces a "message digest" or checksum that can be used to verify data integrity. I realize that "MD5 passwords" are common, but they are not encrypted. What is actually stored or compared is the checksum of the password. Theorectically, only the real password will produce the correct MD5 checksum.

If you look at things like SSL, you'll notice that during the connection negotiation, things like encryption cipher (DES, 3DES, IDEA) and the checksum (MD5, SHA) are chosen during the initial handshake.