A siren and a keypad by the front door is what most people picture when they hear "alarm system", but that is the smallest part of it. The panel, the detectors and how the system tells someone something is wrong are three separate decisions, and the keypad is really just where you talk to the first of them.
The panel is the brain - it tracks which zones are armed, on what schedule, and what counts as a genuine alarm versus a door that was opened during business hours. Paradox panels make up the large majority of what we install, largely because the same panel line scales from a small retail fit-out to a multi-zone commercial site without changing platform.
Detection is where PIR motion sensors and beam detectors do different jobs, not the same job at different price points. A PIR covers a room or an open area - it is what most people mean by "motion sensor". A beam detector from Takex projects a line between two points, which suits a fence line, a driveway or a loading dock entrance where you want to know something crossed a boundary, not that something moved somewhere inside a room. Sites with an external perimeter to cover generally want both, not one or the other.
How the alarm actually reaches someone is the part that is easy to skip past. A monitored system, wired through a Honeywell or Bosch communicator, reports to a monitoring centre that can dispatch a response without anyone on site doing anything. A self-monitored setup sends the alert straight to a phone instead, which costs less ongoing but relies on someone actually seeing and acting on the notification.
Wireless detectors from Inovonics are worth knowing about specifically for retrofits - heritage-listed premises, tenancies where running cable through a shared wall is not an option, or a site that needs a sensor added after the original install without opening up a ceiling. They cost more per point than a wired sensor but remove a cabling job that can otherwise make a small addition disproportionately expensive.
If you are speccing a new system or adding to an existing one, tell us what you are actually trying to cover - a retail floor, a warehouse perimeter, a server room - and we will put a detector mix together rather than starting from a fixed package.