Fuel oil, commonly known as heating oil, is a petroleum-based product primarily used for heating homes and buildings. It is categorized into different grades based on its viscosity and sulfur content, with higher grades being more refined and cleaner burning. Fuel oil is stored in tanks near or inside homes and is pumped into heating systems where it is burned to produce heat. It's a reliable source of warmth in colder climates but is gradually being replaced by cleaner alternatives like natural gas, propane, and renewable energy sources due to environmental considerations.