Firstly: A steam based power generation unit is a combination of boiler and turbine. The boiler generates steam using coal, natural gas or other fuels such as biomass, and the super-hot, high pressure steam runs a turbine to generate electricity. This is pretty much most power plants worldwide work to generate the electricity for you and me.
Let us look a bit more closely at the steam part of this system.
A boiler heats water and generates steam. The high pressure, high temperature steam turns a turbine; having done the useful work, the steam leaves the turbine at a lower temperature and pressure. This steam is condensed into water, which again enters the boiler and the cycle repeats.
The above structure works quite good for a system whose main aim is to generate power.
Now, consider a typical process industry (chemicals, sugar, pharmaceuticals, food…) in which steam is used not for power, but for heating/drying. Owing to the nature of such processes, significant amount of energy in the high temperature, high pressure steam is wasted. So, someone thought, would the overall efficiency of the process be higher if the steam at high pressure and temperature first runs a turbine, generates power, and the steam that leaves the system (at a lower pressure and temperature that is good enough for heating) is then utilized for the original heating purpose?
It turns out that this is an effective idea, and this has given birth to the CHP (combined heat and power) boiler sector.
The turbine used to generate power in this system needs to be what is called the Non-condensing type. That is, after the steam exits the turbine, it does not condense into water as it does in a conventional steam turbine. And of course, it should not condense into water, because the steam has to travel onward for its original purpose of heating or drying!
Such non condensing steam turbines that are used in cogeneration or combined heat and power generation are also called back pressure steam turbines.
I remember some of my visits to companies making these back pressure steam turbines in India. Many of these are small to medium firms, run by entrepreneurs having a background in conventional boilers or steam turbines, but who realized that there was a more efficient way to run these things!