These are usually smart energy assets (digitally or electronically controlled), although not all smart energy assets can be used for demand response.
What do we mean by a flexible energy asset?
An energy asset with production or consumption capacity, the use of which can be flexibly shifted in time 'at will', usually within some constraints.
For example: cooling devices, heating devices, electric vehicle batteries, etc. (flexible consumption assets) - or hydro power plants, large batteries, etc. (flexible production assets).
While the constraints can be quite tight for one such asset, so that e.g. room temperature stays within wanted limits, pooling many such assets will allow more 'combined' flexible capacity.
Wait... - How is that different from renewable energy production?
Renewable energy production from solar and wind power plants is nearly pollution-free, but cannot be shifted in time. Quite contrarily, the production will vary depending on local weather and time of day.
That's why the rising share of less controllable renewable energy production (from all energy production) means that more and more flexibility will be needed in the power system.
However, what flexible energy assets and renewable energy production assets have in common is, both types of assets can be controlled together in an energy management system or a virtual power plant.
What is a virtual power plant?
A virtual power plant (VPP) means digitally enabled aggregation of various smaller energy assets, for the purposes of managing them as a totality, similar to one big asset (of, say, power plant scale).
This is done to enable ‘optimization’ of the use of the assets and of the local electricity grid, and to enable participation in demand response electricity markets.
At best, a virtual power plant can in a way replace a real power plant! See electricity markets page for more about VPPs and demand response.