Geothermal energy is a promising alternative for replacing fossil fuels to ensure the continuity and well-being of human life. Geothermal energy sources have two main categories: high-enthalpy and low-enthalpy energy sources. High enthalpy energy sources are used to drive conventional power generation cycles such as the Rankine cycle. Low enthalpy energy sources can be applied for heating, cooling, seawater desalination, or driving binary power generation cycles. Geothermal energy sources usually are used for driving poly-generation systems to increase profit and reduce exergy destruction in the system. In general, poly-generation systems driven by geothermal energy are established to produce any possible combination of the following: electrical power, heating, cooling, greenhouse heating, water desalination, and hydrogen generation. In this paper, a review of applications of geothermal energy sources is carried out. It was observed that using poly-generation systems leads to more energy and exergy efficiencies than the systems used for only one application. Moreover, applying geothermal energy for water desalination, heating, and cooling processes is a suitable solution for providing the required utility in arid and isolated islands. Additionally, the desalination of geothermal brine is a suitable source for providing potable water in arid regions and islands. Moreover, this brine may drive other cooling and heating processes. Also, from a review of the geothermal energy applications, it was concluded that using geothermal energy is more cost-effective and a good alternative to fossil fuels to generate electrical power in an environmentally clean and sustainable manner.