Straight from the Google
The OS will be updated regularly directly from the Google, in case your device is running vanilla android version. And what makes these features important is the apparent lag between the release of latest android OS and the time we get the updates for the particular smartphone. The S3 I had did not receive an up-gradation to Jelly Bean 3 months after the new OS's official release.
Devices running Vanilla android
And obviously the nexus group of smartphones run this flavor of android. Now we have increasingly number of other devices offering this feature like the new S4 which has a vanilla android version. Rumors are that the HTC One has the same offering in the pipeline.