An ELD is a compliance device. It automatically records driving time from the engine control module and produces the hours-of-service log a driver must present at a roadside inspection. Its scope is the logbook and the data around it — duty status, location, engine hours, miles.
A TMS is an operations system. It is where a load gets entered, assigned to a driver, tracked through pickup and delivery, closed out with a signed proof of delivery, invoiced, and settled. It manages customers, carriers, rates, and documents. It does not, by itself, record hours of service.
The two connect. Many TMS platforms integrate with ELD providers so dispatchers can see available driving hours and location without a second login, and so status updates flow automatically. But an ELD integration is a feature of a TMS, not a replacement for one — and an ELD on its own does nothing to run dispatch or billing.
A one-truck operation can technically run with just an ELD and a spreadsheet. Once there are multiple trucks, repeat customers, or accessorials to track, the spreadsheet is the part that breaks first.
Terms in this answer: Proof of Delivery (POD)
In Trailflow: ELD and other integrations.