There are many factors that are important for the formation of coal. Let take a look at some of them to give you a better idea of what coal is:

..Rapid Growth of Plants
..Types of Plants in the Area
..High and Permanent Water Table
..Land Subsidence
..Deep Burial of Sediments
..Coal Formation Periods

This one is sort of obvious. If you don't have a lot of organic plant material to start with, you just aren't going to get a lot of coal. The thick coal seams of today represent thousands of years of sedimentation. To form one meter of bituminous coal requires
between 6000-9000 years of sedimentation, and many coal seams are many meters thick. Peat, that is organic material saturated with water, is the basic ingredient for making coal, Peat is a mixture of living and dead plant materials, as well as a host of microbes and inorganic materials.

Generally, the deposition areas were regions which had little temperature fluctuation, and were warm, like the Everglades in Florida. However, not all coal seams formed under these conditions. Some formed under more temperate climates.

The types of plants are also important. Grasses and sedges contribute some, but not a lot, of organic material to peat. It is trees that were the major source of organic material. We have to be careful about the word "tree" here. The ancient trees,
especially in the Carboniferous period (~300 million years ago) were quite different than those of today. There are no living representatives of these trees, such as the Lycophyta, Pteridophyta, and Equisetophyta. In some cases, there are recognized
descendents. For example, the nearest living relative to the ancient Lycophyta,is the horsewort, which only gets about two feet tall. More recent coals formed from deposits in the Mesozoic had a closer relationship with modern (Gymnosperms) plants, for
example, (Ginkgophyta, Cycadophyta and conifers). (More detail on the flora can be found at: Paleobotanical Research Group)
and the references therein.

A swamp is needed for coal to form. But not just any swamp. The swamp must not dry out to any great extent. If it does, then the organic material is destroyed by the reaction of oxygen and aerobic bacteria to CO2 and H30. You see dead trees in forests, but they just fall apart and disappear. Without water to cover them
and rapid growth of material on top of them, the organic materials do not survive.

Another condition is that there should no be a rapid flow of water through the peat. Near surface water contains dissolved oxygen, and if there is a rapid steady flow of water, then the peat will again be oxidized and destroyed.

Another important component for coal to form is that the land must continue to subside while the peat is depositing. If the land doesn't subside, then sooner or later the atmosphere and the microorganisms will destroy the biomass. Often the subsidence can be in fits and starts, so that at times the land is rapidly (geologically speaking) dropping. However, if the ground drops too rapidly, or the water
level rises too rapidly, the swamp can drown. Then, instead of organic matter building up, inorganic materials, such as clays, start filling in the swamp. Thats one reason we have coal seams, i.e., layers of coal with rock in between.

The peat that forms at the surface must be buried deep enough so that the heat within the earth can change it to coal. If there isn't sufficient heat, the peat will not "mature" or metamorphose into coal. There are many instances in large coal seams where the coal has matured to different extents in different places, because of undulations in the earths crust. However, the variations are not such that at one point you'll find peat, and at another point you'll find hard coal. It is very difficult to obtain information of how deep a peat was buried, because there are all sorts of tectonic activities that effect the thermal history. However, in many cases, the coals probably reached depths of 2000-5000 meters. The last stage of the coalification process must be a rising landform. If the coal remains buried deep in the earth, it is too expensive to
mine. People only are concerned with coals that have risen from the depths. However, scientists are interested in sampling even the still deeply buried seams. Much can be learned about coalification by examining continuous sequences of these still
evolving coals. 

Coal has always been forming and is still forming now. Like most geological processes, we cannot wait and watch for it to happen. Scientists are trying to model coal formation in the laboratory, but it's not easy to define reasonable conditions which
mimic in several months what nature does in millions of years. Even though coal has always been forming and can be found in any era, there were several periods in ancient history where land and climate conditions were just right to allow vast swamps to form over very long periods. One was during the Carboniferous around 300 million years ago. The second was in the Mesozoic particularly in the Cretaceous and Jurassic periods. In North America, coal was formed in both eras. The coals in the Midwest and Eastern United States were formed during the Carboniferous,
while the coals in the Western United States were formed in the Mesozoic.

Information provided by: http://chemistry.anl.gov