Source: Wikipedia.
Air Unit Load Device
Unit load devices (ULD) come in several dimensions and are designed to fit the bellyhold of a jet plane, particularly wide-bodied aircrafts. The number of containers that can be fitted is a function of the length of the cargo area. The loading of ULDs is rather simple as it involves rolling in the container. 32 such ULD can be loaded in a 747-400. ULDs offer several advantages:
  • They use standardized sizes and equipment that fit in most  commercial aircrafts.
  • Faster handling (loading and unloading) and tracking of the cargo.
The main drawback is that ULDs can only by used for air cargo operations and a transfer to another mode (trucking) requires consolidating or deconsolidating the loads carried.