Cisco IOS: What Is The Difference Between The 12 Firmware and 15 Firmware

I have had to look at this in the past and have come back across this recently, but what are the differences between the 12 code and the 15 code?  It depends on the model of router you are looking at.  With the newer router series (1900, 2900, and 3900 routers), licensing and one code is the deal.  You get one universal code and you have to have licenses to unlock features and functionality.  Its a little easier to understand and it sounds a lot like Check Point, which is NOT easy to understand to me.  The universal code has all options available in the code for base, security, voice, and enterprise level features, but you have to license them.  All in one package.  So when you go to download an upgrade for the 1900, 2900, or the 3900, you wont see several IOSs to download.  Just one universal image.  Make sure you license for what you need.
For the 1800, 2800, and 3800 models, you still get the various names in the IOS.  c2801-adventerprisek9-mz.151-4.M6.bin or c2801-advipservicesk9-mz.151-4.M6.b are examples of this.  At this point, you just dont see any universal images for these models, and I doubt you ever will.  But, you don't have to have the licenses on these models.  Just download the image you need for the features you need and do the upgrade, even though it is the 15 code.
With all of this said, there are obviously going to be feature and functionality enhancements.  You will have to look over the release notes for the image version you want to see the updates in code for more info on that.  For example, I know from 12 to 15, there are some SIP security 'features' that when you do the upgrade, SIP will stop working to your carrier if you don't plan ahead of time for the correct configuration changes. So read your release notes.