What is DRPM and How does it differ from RPM?

A drpm stands for delta rpm, which is an addition to an existing rpm, and only contains the different files. Source:

Delta RPM packages contain the difference between an old and a new version of an RPM package. Applying a delta RPM on an old RPM results in the complete new RPM. It is not necessary to have a copy of the old RPM, because a delta RPM can also work with an installed RPM. The delta RPM packages are even smaller in size than patch RPMs, which is an advantage when transferring update packages over the Internet. The drawback is that update operations with delta RPMs involved consume considerably more CPU cycles than plain or patch RPMs.

The README file referred to in the documentation can be found in the GitHub repository. You will see that deltarpm is based on bsdiff.

DRPM files are Delta RPM files. They do not contain a complete package but only the difference (= delta) between two (RPM) packages. This is useful for faster download and installation of RPM files that are an upgrade to an existing (installed) package. Only new and changed files need to be included here.

In order for yum to work with drpm files you need to install the deltarpm package:

yum install deltarpm

This contains the necessary tool.

See also Do I need to do something about "Delta RPMs disabled"?

DRPMS: Instead of downloading full packages for minor changes , delta rpms were invented. As far as I know from Suse linux first. Only changes between the installed package and the new one are downloaded. Once the delta rpm loaded, a rebuilding process is started. At the end, the reconstructed RPM is put into yum download directory, where the normal package installation routines pick it up and install it.