In a steam cycle with a steam turbine the fossil fuel is burned in an oven and produces heat. A coil fed with water goes into the oven, then the water evaporates and produces superheated steam. The superheated steam is then expanded in a steam turbine which produces electricity.
In a gas/steam combined cycle with a gas turbine (turbogas), the fossil fuel is burned in an oven and the combustion gases exiting from the oven are firstly expanded in a turbogas that produces electricity and then pass into a boiler where they produce steam. The steam goes into the steam drum, the economiser and the superheater and is then expanded in a steam turbine to produce more electricity.
The gas/steam combined cycle can also be linked to a steam cycle.
In cogeneration with turbogas the process is quite similar to a combined cycle, the only difference being that the steam is used in other industrial processes.
Many oil, petrochemical and chemical processes produce effluents that can be used as fuel for burning in ovens to produce exhaust gases which can be expanded in a turbogas, therefore resulting in a combined cycle or cogeneration.
A steam cycle with steam turbine does not use a steam drum because the steam has already been superheated.
On the other hand, a gas/steam combined cycle and cogeneration uses a steam drum to protect the superheater.