Just the tractor — the carrier supplies the truck and driver, and the shipper or broker supplies a pre-loaded trailer. Common with drop-and-hook freight and retail distribution networks.
In a power-only arrangement, the carrier shows up with just a tractor and driver — no trailer of its own — and hooks onto a trailer the shipper or broker already has loaded and staged. That works well for drop-and-hook freight, where a driver picks up a preloaded trailer and drops an empty one without waiting for a live load or unload, and it's become more common as retail distribution networks have built out their own dedicated trailer pools.