This is a great video showing how to make a rocket stove out of four two-hole concrete blocks.  The arrangement is straightforward and quite ingenious.

A rocket stove uses kindling sticks to burn much hotter they they would normally allowing you to cook using very little fuel, generating a great deal of heat at the cooking surface.  In fact it is so efficient, it can operate on about half as much fuel as an open fire, meaning a useful amount of heat can be generated from what would normally only be regarded as kindling.  I found out about Rocket Stoves thanks to an article which can be found over at Bugged Out Prepper.  There is also the obligatory Wikipedia article which gives some further details:

“In field tests in India, rocket stoves used 18 to 35 per cent less fuel compared to the traditional stoves and reduced fuel used 39-47 per cent compared to the three stone fire, as well as a large reduction in emissions”

In the UK, most concrete blocks you see are the dense, solid ‘breeze blocks’ and the blocks similar to those used in the video are called hollow blocks, an example of which can be seen at Travis Perkins.  There is also a brand called Masterblock Masterdenz.