The Earth Cargo Service (or ECS) was the primary operating agency governing the operations of Earth-registered interstellar cargo transport vessels in the first half of the 22nd century. It was governed by the Earth Cargo Authority. Ships needed a license to operate under the ECS, which in some cases could be lost if a rendezvous was missed. (ENT: "Broken Bow", "Fortunate Son", "Horizon")
The crew of these ships would spend years traveling from one planet to another; whole generations of these "boomers" grew up in space, and the ownership of ships often passed down family lines.
Among the planets serviced by the Earth Cargo were Draylax, both Teneebian moons, Trillius Prime, Verex III, Vega colony, Deneva colony, and presumably Earth. Trade also existed with the Demerians and the Orions.
The ECS starships were frequently attacked by marauders like Nacicans, Klingons, Orion Syndicate etc. Often times ECS starships had to defend themselves and take heavy losses and Earth Starfleet had problems of its own. Lot of powers in the Alpha Quadrant never took Earth seriously.