Dictionary of Occupational Terms Based on the Classification of Occupations used in the Census of Population, 1921 provides a description of what that occupation involved — sometimes a brief single phrase, in other cases running to several hundred words. There is extensive cross referencing, with listing of synonyms and similar occupations in related spheres of employment, and it includes a considerable number of purely regional terms. In all, it provides 16,837 definitions for 29,106 terms.
Originally compiled by the British Ministry of Labour and published by HMSO in 1927 it was based on the classification of occupations used in the 1921 Census, this edition was scanned and formatted for the web by Peter Christian, author of The Genealogist's Internet and co-author of Census – The Family Historian's Guide
This digital edition of the Dictionary includes the complete set of definitions, the prefatory material, and an independent index not based on the index of the original volume.
This description is based on a post on the Society of Genealogists Rootsweb mailing list.
You may well find this reference useful for occupations in earlier censuses, 1921 is not yet available at the individual level. Statistical compilations are available online such as at www.visionofbritain.org.uk/census/census_page.jsp?yr=1921