Here is a recent HBS working paper by Christensen and others. He clarifies the concept:
Sustaining innovations - when technological change occurs along dimensions that main stream customers care about and that incumbents have provided in the past.
Disruptive innovation - the new innovation does not improve the attributes on which existing firms built their business (different trajectory/new 'unique' combination of product attributes - could in some ways be perceived to be inferior to existing product)
Technological change outstrips growth in demand for 'higher performance', So incumbents over-serve the market in terms of product features/attributes (consumers don't really want them) and this also leaves some room at the lower end of the quality scale for entrants (mini-mill story).
Incumbents don't see the attractiveness of investing in 'inferior' product with lower margins (and this is also why they are not a source of disruptive innovation). Or we could use Ghemawat's argument about commitment to explain why this does not happen.
All disruption does not have to be 'inferior' - new market disruptions "take hold in a new value network" and compete against 'non-consumption' - example: Godrej chotuKool portable refrigerator.