Fuel cells can be attributed to electrochemical power source. Basic science and other useful information about fuel cells are provided in this book, which helps for better understanding of the problems of fuel cells. This is also important for understanding the terminology in case of fuel cells, since they are very important parts of electrochemical devices. Some improvements of fuel cells are mentioned in order to consider them as the promising technology for transportation and energy generation. The processes, classification, advantages and disadvantages of fuel cells are described here. Some additional information about more terminology, such as gas diffusion layer, catalyst, bi-polar plates are included and explained clearly.