Greenhouse gases are the gases that absorb long-wave energy and emit it back into our atmosphere. They are responsible for keeping the earth warm enough to live on. Most of these gases are present in the atmosphere naturally. However, anthropogenic (manmade) processes are dramatically increasing the concentration of these gases. This is one of the main reasons we think our earth is experiencing warming and climate changes.
Concentrations of greenhouse gases are commonly given in percentages as well as mixing ratios of gases to total air volume, such as ppt, ppb, and ppm. The percentages are the percentage of the atmosphere made up by these gases. Since the percentages are very small, most of the time concentrations are referred to in parts per trillion (ppt), parts per billion (ppb) or parts per million (ppm). This means that for every trillion, billion or million molecules, the number given is the amount of molecules that are composed of that gas per trillion air molecules, etc. For example, as of 2009, our atmosphere currently contains a CO2 concentration of 385ppm; for every million molecules, about 385 of them are carbon dioxide.
In the linked pages, you should also keep in mind that a fossil fuel is any hydrogen and carbon rich substance that was created by the decomposition of prehistoric plants and animals that can be burned to produce heat or energy. This includes coal, petroleum, and natural gas products.