Electricity from renewable resources
Energy resources are divided into renewable and non-renewable based on the timescale for them to naturally replenish. A resource that regenerates on a shorter timescale than a human lifetime is labelled as renewable (wind, for example), while others are non-renewable (crude oil, for example).
Renewable vs Sustainable
It is noteworthy that renewable and sustainable are distinct concepts. Sustainability is related to the way humans are exploiting resources, not to the nature of the resource itself. For argument's sake, while all biomass resources are renewable, excessive or poorly organised exploitation can make it unsustainable.
Most renewable resources can be converted into electricity, notably sun radiation, wind, water flow (rivers or artificial waterways), oceanic motion (tides and waves), geothermal heat, as well as biomass. There is not necessarily only a single way to convert a resource into electricity. For example, sun radiation can be converted with photovoltaic panels or concentrated solar powerplants.