kernel versioning in RHEL: what are the differences between minor versions of the rhel kernel?

How can I determine what the differences are between minor revisions of the kernel package?   For example, we have 2 servers built on the latest rhel 5 x86_64 available that were built several months apart.  The first is rhel 5.6, and has

kernel-2.6.18-274.el5 , the second is rhel 5.7 w/ kernel-2.6.18-274.12.1.el5.   What does the extra "12.1" mean,  and what would we loose if we replaced it with kernel-2.6.18-274.el5 ?