Smoke alarms are installed in the bedrooms and hallways. This is a safety feature required by the National Electrical Code.
Newer homes have smoke alarms that are wired to the main electrical supply and are also tied together so that if one alarm goes off, they all go off. These alarms also have the 9 volt battery back up.
You should change the batteries every 6 months.
You should remove the cover and vaccum the alarms also.
Alarms do not last forever, so you should replace them at least every 10 years.
Carbon Monoxide detectors are also recommended in all homes that have gas appliances.