IRM is a great way of protecting information from unauthorised use by keeping the access policy with the information rather than where it's stored. For example, I can restrict who can copy and paste, forward and print an email. I can say who can read and update a document. And this all works inside and out the firewall. If you think how you traditionally protect a document, you probably make it read only or make the share it's stored on read only. That doesn't stop me taking a copy of the document and forwarding it either accidentally or maliciously to someone internally or externally.
With RMS I can restrict the document so that some people have Read Only access to the document, some have Read Write and everyone else has no access. I can also set things like expiry dates on the document. And the great thing is, that security stays with the document. So if the document gets forwarded then the security stays in place.
To find out a bit more information about IRM and RMS then go to http://www.microsoft.com/technet/prodtechnol/office/office2003/maintain/rmsirm.mspx
A great way of getting up to speed with this Technology is to go to the Technet Virtual Labs where you can try out setting up a server and client and test out using Office 2003.
There's also some "How To" documents at http://www.microsoft.com/resources/documentation/windowsserv/2003/all/rms/en-us/InitExp_5.mspx