Jim,
It is an annual subscription. You first get a set of CDS, about 5, that includes all of the operating systems, all of the developer's tools, the entire office suite, and a bunch of other stuff. Then you get Quarterly updates that either add cds toi the set or replace cds you already have. You get a number of support incidents without charge. You get prerelease version of stuff sent to you.
If you were to add up the cost of everything in the st, it exceeds the cost of the subscription. However, if you don't need all the stuff then it would be of questionable value. I, personally, find it indespensible. I cna test on any os version there is. I don't have to go chasing and downloading SP's, they just come in the mail.