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