RI is the base architecture implementation for J2EE. Base on that, other vendors implement their own J2EE based frameworks, i.e. Sun offers RI for free which you can get from their website, that is RI, on the other side vendors like, Oracle, Borland, IONA, etc have their own implementation of the framework. in simple words, all it is, is the standard implementation of the framework and the rest are superset of RI.