Both micro hydro and small hydro are types of hydroelectric power, and the only difference between them is the generated capacity, with micro hydro having a capacity of up to 100 kW while small hydro referring to hydroelectric projects with a capacity of up to 10 MW.
The upper border for small hydro isn't the same in all countries across the globe, in United States for instance small hydro refers to all hydropower projects with a generating capacity of up to 30 MW, while in Canada it stretches all the way to 50 MW.
Micro hydro is really the subtype of small hydro and its main purpose is to provide power to an isolated home or small community, sometimes by complementing other renewable energy sources such as solar power.
In most cases micro hydro installations do not have a dam and reservoir like this is the case with larger hydroelectric projects.
Both small and micro hydro projects are usually built in isolated areas where it is not economically viable to serve from an electrical network, or in areas where there is no national electrical distribution network.
Small and micro hydro systems have relatively low environmental impact compared to large hydro, not to mention much lower construction costs. The developers of small hydro projects still need to make sure not to negatively affect the health of the stream. There's also much simpler licensing procedure compared to large hydro projects.
Small hydro systems, particularly micro hydro systems are very flexible and can be installed in a number of different environments though their current locations primarily refer to rural isolated areas.
The country with the largest number of small hydro systems is China. Small hydro is China's main clean energy option to provide electricity for tens of thousands villages all across the country.