ARV's (Axiom Return Vehicles) were long-range spacecraft built by the BnL Corporation and tasked with delivering EVE units to Earth in order to scan for lifeforms. Probe ships used a laser scanning system in order to find firm, level ground on which to land. The Axiom has two docking bays for ARV's (or Probe Ships).
Once an EVE unit found an example of life, the ship was programmed to retrieve the unit and return it to the Axiom. WALL•E hitched a ride on one of these ships, as he had grown quite attached to this unit and couldn't resist being without it.