Machine-to-machine describes devices exchanging data with each other or with a central server automatically. The classic examples come from the cellular world: a vending machine reporting stock levels, a fleet tracker sending GPS fixes, an alarm panel dialing a monitoring center. Each used a SIM and a point-to-point link to one backend, usually run by the same company that owned the machines.
The Internet of Things grew out of M2M and widened it. Where M2M systems were typically closed, vertical and tied to a single application, IoT assumes internet protocols, shared platforms and data that can be combined across sources. The distinction is partly historical, and carriers still sell “M2M plans”, but it is a useful reminder that cellular IoT has decades of deployment behind it.
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