I had a need to copy some specific RPM files locally to my machine, but have the general YUM database recognize them (not using yum localinstall.) I found this lovely howto that explains how to do it.
In my case, I created a folder for one RPM I wanted in the local yum repository. I then installed the createrepo package, used it on my new directory containing my RPMs, then added a repository file pointing to the new local repository.
cp <DESIRED RPM FILES> yumlocal
yum install createrepo
The last piece was to create a yum repo file local.repo
[local] name=CentOS-$releasever - local packages for $basearch baseurl=file:///path/to/yumlocal/ enabled=1 gpgcheck=0 protect=1
That was it! Now I could use yum install <NAME OF PACKAGE IN LOCAL REPO FILE> and it works!