A CRM claim is also called a Beauregard claim after the 1995 case that established it as an allowed approach to organizing a claim. The goal is to make a CD (for example) siting on a shelf an infringing article of manufacture. A method claim requires execution of steps, a system claim requires hardware but a CRM claim is infringed by the media with bits on it. Not any bits, but bits that are instructions to a machine to carry out the steps of what would (if organized differently) be a patentable method claim.
A non-transient computer readable medium containing program instructions for causing a computer to perform the method of:
do step 1
do step 2
do step 3.
The "non-transent" limitation is due to a case several years ago that ruled a disembodied signal was not a "thing". The bits have to reside somewhere other that flying through the air or down a wire.
Sticking with CD's, just stamping them out would be a direct infringement of a CRM claim but not of the equivalent method claim or system claim.