What is biochar and what are its benefits?

Biochar Enhances Crop Yield, Enriches Soil & Protects Water. ... Nitrogen tends to run-off regular soils, upsetting ecosystem balance in streams and riparian areas. Biochar also holds gasses; recent research has proven biochar-enriched soils reduce carbon dioxide (CO2) and nitrous oxide (N2O) emissions by 50-80%.

So what is the cheapest way for an urban dweller to make their own biochar?