Historical Thesaurus data consists of a fine-grained conceptual hierarchy containing almost all of the recorded words in English, arranged semantically. This hierarchy of semantic categories is unique in its depth and level of detail, consisting of almost a quarter of a million concepts. Each category is then nested within other, wider categories – so that, for example, the category of words meaning Profligacy, dissoluteness and debauchery is within Licentiousness, itself adjacent to Guilt and Wickedness and within the major category Morality.
This hierarchical structure differs from the organization of many other thesauri; Historical Thesaurus categories relate to others not just linearly, but can operate either horizontally (on the same hierarchical level) or vertically (on a higher or lower level, either containing or being contained by another category). In addition, each concept is able to contain a series of subcategories within itself, separate from the main sequence. You can see this on any category page on this site, which each have navigation options to travel "Up the hierarchy", "Down the hierarchy", a set of subcategories (if present), and links to move horizontally to other parts of speech which refer to the same concept. Choosing to go up the hierarchy and scrolling down to the bottom of a category page can let you see the neighbour categories of any given concept.
The three primary divisions of the Thesaurus are I The External World, II The Mental World, and III The Social World. These in turn are divided into 377 major categories, such as Food, Thought, or War. Each category is therefore given a nested reference code, or hierarchy number, such as "01.07.02.02.06.01 n" for the category Whisky. Each new set of two digits indicates another level of depth in the hierarchy, from 01.07.02.02 n Intoxicating liquor all the way upwards to 01 n The world. More about the principles of classification and editorial policies of the Thesaurus can be found in the Unlocking the OED: The Story of the Historical Thesaurus document here. A comprehensive series of changes have been made to the Thesaurus hierarchy since its first publication, and category numbers online may not correspond to those in the printed version – more information is available here.
You can also download the first three levels of the Thesaurus hierarchy as a PDF here.