The main reason why fossil fuels cause so much damage is due to the amount of carbon dioxide or CO2 emitted when fossil fuels are burned. This burning and release of CO2 is a huge contributor to the greenhouse effect and climate change. More CO2 in our atmosphere causes less sunlight to escape back into space, causing the planet to warm.
When fossil fuels are burned, CO2 isn't the only gas released. Other harmful gases such as nitrogen and sulfur oxides. Burning fossil fuels causes air pollution and smog.
Geothermal energy is clean because it can be generated without burning fossil fuels. Geothermal energy is considered renewable because the heat is continually replaced. The water that is removed is put right back into the ground after its heat is used.