The British government took its first national census in 1801. A census has been taken every ten years since that date, except in 1941. The first genealogically useful census was not taken until 1841, when names were recorded.
The 1901 Census for Wales was taken on the night of 31 March 1901. The following information was requested: Name of street, avenue, road, etc.; house number or name; whether or not the house was inhabited; number of rooms occupied if less than five; name of each person that had spent the night in that household; relationship of person enumerated to the head of the family; each person's marital status; age at last birthday (sex is indicated by which column the age is recorded in); each person's occupation; whether they are employer or employee or neither; person's place of birth; whether deaf, dumb, blind, or lunatic. Some of this information may be recorded in Welsh or English.
Enumeration forms were distributed to all households a couple of days before census night and the complete forms were collected the next day. All responses were to reflect the individual's status as of 31 March 1901 for all individuals who had spent the night in the house. People who were traveling or living abroad were enumerated at the location where they spent the night on census night. All of the details from the individual forms were later sorted and copied into enumerators' books, which are the records we can view images of today. The original householders schedules from 1841 to 1901 were destroyed.
Census returns were collected according to registration district. These returns were divided into sub-districts and assigned consecutive piece numbers for reference purposes. The piece numbers begin in London with number one and work roughly south to north, followed by the Welsh districts and then the Isle of Man and Channel Islands. You will find the piece number on a paper strip at the bottom of every image, following the PRO class number. There may be hundreds of pieces within a county.
In addition to the piece number, each page of the returns includes a folio number and/or a page number. The folio number was stamped onto every other page before microfilming and is located in the upper right hand corner of the image. Folio numbering usually starts over at the beginning of each piece. The page number is part of the printed form and is found on every page in the upper right hand corner. The page numbers start over at the beginning of every enumeration district. A full reference number for a record in the 1901 census includes the PRO class number (RG13), the piece number, the folio number, and the page number. Keep in mind that you may have to look at several enumeration districts to find the page you want within a given folio since the page numbers start over with every ED.
The clerks who compiled and reviewed the census data made a variety of marks on the returns. Unfortunately, many of these tally marks were written over personal information and some fields, such as ages, can be difficult to read as a result. More useful marks include a single slash between households within a building and a double slash separating households in separate buildings.
Taken from "Chapter 6: Census Returns," Ancestral Trails: The Complete Guide to British Genealogy and Family History by Mark D. Herber (Baltimore, MD: Genealogical Publishing Co., Inc, 1998) and Using Census Returns, Pocket Guides to Family History by David Annal (Richmond, Surrey: Public Record Office, 2002).