Hydro employs the simple conversion of mechanical energy into electrical energy using turbines (generators).

Solar energy can be converted to electricty in two ways: heat is converted to mechanical energy and then to electricity in a manner similar to hydro, and light energy is converted directly into electricity by photoelectric means.

Hydroelectric dams work because the water held behind the dam is being let out slowly to the other side. While the water moves through pipes to the other side, the force of the water flowing spins a turbine. That turbine has magnets placed so that when the turbine spins it generates a magnetic field.

if you are talking about dams, than water passes through turbines which is basically a big underwater fan. as the water passes across the blades it makes it spin. the turbine is connected to a generator which makes electricity.

Solar plants. if your referring to oraganisms i cant help but a plant that doesnt use solar panels probably uses mirrors that are pointed at central point, usually a tower. All this light shines on the tower heating water until it boils. The hot, pressurized steam rushes over a turbine just as at the dam, and that runs a generator to make electricity.