In a nutshell A composting toilet is a toilet which uses the process of composting to help break down human waste into a nutrient rich substance called 'humus'.
To ensure the composting happens quickly, the toilets try to provide the ideal conditions for the microorganisms - (microbes, bacteria & funghi) that do the hard work; specifically - food, oxygen, moisture (but not too much) and the right acidity.
Everything needs food and microorganisms eat carbon-rich organic (natural) foods. Because of this, we initially add a small amount of natural carbon-rich material such as coco coir to the toilet. This also facilitates regulating the moisture content which we'll come to shortly.
Excess liquid from urine can surround the material and prevent the microorganisms having enough oxygen. To solve this problem, compocloset toilets separate pee at source making them urine-diversion composting toilets. Poop also has a slightly high moisture content for effective composting so our toilets include a fan to circulate air and a carbon filter to remove any odours.
Human pee has high levels of ammonia which has a low pH level which slows down the work of the microorganisms. By separating the pee, the microbes have a happy environment to do their important work in.
To Summarise... A composting toilet is a toilet where we help nature to help us by providing the optimal conditions for microorganisms to compost our waste!