Bio-mass and hydro-electricity both are renewable sources of energy.
Bio-mass is organic matter used as a fuel, especially in a power station for the generation of electricity. It is obtained from dead plants and animal wastes. Therefore, it is naturally refilled. It is the result of natural processes. Wood, gobar gas, etc. are some of the examples of bio-mass.
Hydro-electricity, on the other hand, is acquired from the potential energy stored in water at a height. Energy from it can be produced again and again. It is harnessed from water and obtained from mechanical processes.