What is a disruptive innovation?

In the history of literally every industry, the original technologies that shifted the industry from a cottage to more modern, were very complicated an expensive.
For example,
Telegraphs were very expensive and complicated. They had to be run by an operator who you paid to run it.

Mainframe computers were extremely expensive and complicated and had to be run by PhDs.

Then a new technology comes into the fray that is so inexpensive and so simplified that it empowers a new group of user that didn't have the money or the skills to use the incumbent product to use this disruptive offering.

For example,
Toyota versus GM.
Honda 50 motorcycles.
Skype versus established telcos.
IKEA's furniture model.
Portable radios for teens in the 60s.


This is then delivered using a cost effective disruptive business model.