Geothermal energy (earth's thermal energy in form of heat generated and stored in Earth’s core) is renewable source of energy because earth’s heat is practically inexhaustible source of energy that is being replenished by radioactive decay of minerals at a rate of 30 TW.
The Earth's inner heat primarily comes from a heat produced through radioactive decay (around 80%) while the rest comes from residual heat from planetary accretion (about 20%).
Together with being renewable energy source geothermal energy is also considered to be sustainable source of energy because any projected extraction of geothermal resources is small compared to the Earth's heat content (with the worst thing that could happen being the local depletion of geothermal resources).
Renewable energy source doesn't have to be necessarily environmentally friendly source of energy, though in almost all cases renewable energy sources are much more environmentally friendly than currently dominant fossil fuels. Geothermal energy is generally considered to be environmentally friendly source of energy with very few environmental risks when compared to other renewable energy sources.
Despite the fact that geothermal energy is renewable and practically inexhaustible source of energy, world is still using very little of total geothermal energy potential, mostly because with currently available technologies only areas near the tectonic boundaries can be profitably exploited.
Unlike solar and wind geothermal energy is renewable energy source that doesn't suffer from intermittency issue and is available 24-7, throughout the entire year. The capital costs of building new geothermal power plants are still high but once completed these power plants are highly reliable and have low maintenance costs.