MD5 is one of the most common types of hash used to verify the integrity of a file after a transfer (and not only).
You may even find some files with .md5sum extension which contain the MD5 checksum of some existing files.
The MD5 hash is commonly expressed as a 32 digit hexadecimal number, e.g.: 1bc29b36f623ba82aaf6724fd3b16718
A simple way to generate this, is to use the md5sum command:
[root@bubble ~]# md5sum httpd-2.2.15-29.el6.centos.x86_64.rpm f7de6883d5c12e7565955fbccb1b64ef httpd-2.2.15-29.el6.centos.x86_64.rpm
You can also generate .md5sum files yourself with the same command by just redirecting the output into a file:
md5sum httpd-2.2.15-29.el6.centos.x86_64.rpm > httpd-2.2.15-29.el6.centos.x86_64.rpm.md5sum
-rw-rw-r-- 1 root root 822K Aug 13 13:32 httpd-2.2.15-29.el6.centos.x86_64.rpm -rw-rw-r-- 1 root root 72 Jan 30 00:44 httpd-2.2.15-29.el6.centos.x86_64.rpm.md5sum
Of course, you can also use the checksum file to verify the integrity:
[root@bubble ~]# md5sum -c *.md5sum httpd-2.2.15-29.el6.centos.x86_64.rpm: OK