There are three main types of fossil fuels:
(1) Oil and its derivatives
(2) Natural Gas
(3) Coal
Fossils fuels are typically composed of compounds containing hydrogen and carbon atoms.  The process of burning fossil fuels always results in carbon dioxide being emitted into the atmosphere.  Take the example of natural gas (methane).
CH4 + 2O2            CO2 + 2H2O
Water and carbon dioxide are the by-products