The dividing line is roughly how much of a trailer the freight fills. Up to about six to twelve pallets, or a few thousand pounds, LTL is usually cheaper because you only pay for the space and weight you use. Above that, a full truckload is usually cheaper and always faster.
LTL freight moves hub-and-spoke: local pickup, terminal, line haul to another terminal, terminal, local delivery. Every handoff adds time and a chance for damage or a reweigh/reclass dispute. Truckload skips all of that — one driver, pickup to delivery.
Pricing works differently too. LTL is rated on freight class (density, stowability, handling, liability), weight, distance, and accessorials like liftgate or residential delivery. Truckload is a linehaul rate for the lane plus fuel surcharge and accessorials.
Between the two sits partial truckload: too big for economical LTL, too small to justify a full trailer, moved with one or two other shipments and no terminal handling.
Terms in this answer: Less-Than-Truckload (LTL), Full Truckload (FTL), Linehaul, Accessorial Charge