Domestic smoke alarms are designed for use in private dwellings to detect and give warning of the presence of smoke, indicating that a fire is developing. They are small units, easily attached to the ceiling (preferably) or high up a wall.
Single 9 volt battery-powered smoke alarms are the most common and the cheapest but are not recommended. Better options are mains powered with battery back-up or units with a long-life battery.
Interconnected smoke alarms are a worthwhile upgrade. They improve the chances of early notification, because they all make noise when the first alarm detects smoke.