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.