I take the concepts of sustaining and disruptive innovation from Clayton Christensen whose book, The Innovator's Dilemma, I highly recommend. Like all good concepts, the related terms are used to mean other things by other people. Here's what I mean:
I typically use "sustaining innovation" to mean innovations that can be incorporated into the existing practices of organizations including innovations that disrupt standard modes of practice but that can be integrated into the already existing structures of dominant players.
"Disruptive innovations" are disruptive in an even deeper way than simply forcing changes in existing practices in that they disrupt existing models of success. Disruptive innovations are initially inadequate to the jobs performed by current solutions. Perhaps worse for those with a currently successful business model, disruptive innovations require different models to succeed, models which dominant players are unable to effectively pursue because their skillsets are based on past success rather than emergent realities.
For example, the call quality of mobile phones meant that they were inadequate to many of the needs of fixed line callers but for those who needed mobility, poor call quality was an acceptable trade-off to be able to conduct business or keep up with one's friends.
Over time the quality of mobile phone networks improved but, by the time mobile phones became a threat to existing landlines, incumbent players were too far behind to catch up. Though the full story is more complex and landlines have not disappeared, their role is shrinking at a rate that no one would have expected even a few years ago.
Disruptive innovators perform like stealth companies in plain view. Everyone can see them yet those who are threatened cannot see what they are until it's too late. Such shifts may take years to become overnight successes but suddenly everything will have changed and brands that seemed all powerful will be laid low.
A recent example in education has been the uptake of Google Apps for Education as an alternative to Microsoft Office. At first, nobody would have thought free online services could replace Microsoft Office, given the software's incredible ubiquity and complex development, but it's happening.
An example still in the "there's no way that will every be a threat" category is the University of the People (UoPeople), described as the "world’s first tuition free online academic institution". The UoPeople has gathered some serious support and, from what I recall but can't currently find online, their first entering class had a sizable percentage of students from the U.S.
Whatever you think of such an institution's potential threat to established universities, consider the fact that this school does not have a library or employ librarians and does not appear to offer a separate course in information seeking skills. Since disruptive innovations succeed when key chunks of soon-to-be outdated business models are removed, it's incumbent on librarians to take a proactive stance, one that goes far beyond simply finding new allies or incorporating new technologies within a structure that may well collapse unexpectedly.