The phrases are used interchangeably and they describe different things. A smoke alarm is a self contained device that detects and sounds in one unit. A fire alarm system is a designed installation of detectors, call points, sounders and a control panel, designed to a category and commissioned and certificated as a system. Which one a building needs follows from what it is.
The device
A smoke alarm contains the sensor, the sounder and the power in one housing. Domestic and small premises detection is built from these, interlinked so that they sound together. They are covered by the domestic part of the detection standard, which sets out grades by how they are powered and monitored, and categories by how much of the dwelling they cover.
The system
A fire alarm system separates detection from indication and control. A panel monitors every device and circuit, reports faults, decides what sounds where, and can drive other equipment: door releases, smoke vents, lift homing, plant shutdown. That capability is the reason non domestic buildings have systems rather than devices, and it is why the panel is where a duty holder looks first.
Which standard applies
The commercial part of the detection standard covers designed systems in non domestic premises and sets out the categories, the design, the installation, the commissioning and the servicing. The domestic part covers dwellings and is written around alarms and grades. Houses in multiple occupation and small mixed use buildings sit between them, and which applies is a design decision that should be recorded.
Why the confusion costs money
A building told it needs a fire alarm and quoted for interlinked alarms has been quoted for something that cannot drive its door releases or report a fault. A dwelling quoted for a panel system has been sold something it does not need. Asking which standard and which category a quotation is against separates the two immediately.
Questions people ask about fire alarm vs smoke alarm
Does a small shop need a system or alarms?
It depends on size, layout, occupancy and whether anybody sleeps there. The fire risk assessment decides, and many small premises are adequately served by a manual system with sounders.
Can domestic alarms be used in a workplace?
In very small, simple premises they are sometimes accepted as part of the arrangements. In anything with a designed escape strategy, a system is what the strategy needs.
What about HMOs?
HMOs commonly use a graded system under the domestic standard, chosen for the size and layout of the house, and licensing conditions frequently specify the grade and category.