A CTU is any unit that is packed once and travels closed across several modes: a sea container, a swap body, a semi-trailer or a wagon. The concept matters because good-practice rules depend on the unit, not on the mode.
The CTU Code, published jointly by IMO, ILO and UNECE, is a code of practice on how to distribute weight, lash and block the cargo. It is not law by itself, but it is the reference authorities and insurers use to judge whether a packing job was sound.