At The 1940 U.S. Census Community Project you can learn more about the 1940 Census and choose a state to help index (if you'd like) beginning on April 2, 2012.
Short and sweet: here's what you'll find when in the 1940 census (if the census taker followed the instructions, filled every box, and had legible handwriting).
--- Population Schedule ---
1 Street, avenue, road, etc.
2 House number
3 Number of household in order of visitation
4 Home owned (O) or rented (R)
5 Value of home, if owned, or monthly rental, if rented
6 Does this household live on a farm? (Yes or No)
7 Name of each person whose usual place of residence on April 2, 1940, was in this household
8 Relationship of person to the head of the household, as wife, daughter, father, mother-in-law, etc.
9 Sex --Male (M), Female (F)
10 Color or race
11 Age at last birthday
12 Marital status
13 Attended school or college any time since March 1, 1940? (Yes or No)
14 Highest grade of school completed
PLACE OF BIRTH. If born in the U.S., give State, Territory, or possession. If foreign born, give country in which birthplace was situation on January 1, 1937. Distinguish Canada-French from Canada-English and Irish Free State (Eire) from Northern Ireland
16 Citizenship of the foreign born
RESIDENCE, APRIL 1, 1935
17 City, town, or village having 2600 or more inhabitants. Enter "R" for all other places.
19 State (or Territory or foreign country)
20 On a farm? (Yes or No)
PERSONS 14 YEARS OLD AND OVER--EMPLOYMENT STATUS
21 Was this person at work for pay or profit in private or nonemergency Govt. work during week of March 24-30? (Yes or No)
22 If not, was he at work on, or assigned to, public EMERGENCY WORK (WPA, NYA, CCC, etc.) during week of March 24-30? (Yes or No)
23 Was this person SEEKING WORK? (Yes or No)
24 If not seeking work, did he HAVE A JOB, business, etc.? (Yes or No)
25 Indicate whether engaged in home housework (H), in school (S), unable to work (U), or other (Ot)
26 Number of hours worked during week of March 24-30, 1940
27 Duration of unemployment up to March 30, 1940--in weeks
OCCUPATION, INDUSTRY, and CLASS OF WORKER
28 Occupation. Trade, profession, or particular kind of work
29 Industry. Industry or business
30 Class of worker
31 Number of weeks worked in 1939 (Equivalent full-time weeks)
32 Amount of money wages or salary received (including commissions)
33 Did this person receive income of $50 or more from sources other than money wages or salary? (Yes or No)
34 Number of Farm Schedule
SUPPLEMENTARY QUESTIONS (which will be asked of only two individuals per page)
PLACE OF BIRTH OF FATHER AND MOTHER
If born in the United States, give State, Territory, or possession. If foreign born, give country in which birthplace was satuated on January 1, 1937. Distinguish Canada-French from Canada-English and Irish Free State (Eire) from Northern Ireland
36 Place of birth of Father
37 Place of birth of Mother
Mother Tongue (or Native Language)
38 Language spoke in home in earliest childhood
Veterans. Is this person a veteran of the U.S. military forces; or the wife, widow, or under-18-year-old child of a veteran?
39 If so, enter "Yes"
40 If child, is veteran-father dead? (Yes or No)
41 War or military service
42 Does this person have a Federal Social Security Number? (Yes or No)
43 Were deductions for Federal Old-Age Insurance or Railroad Retirement made from this person's wages or salary in 1939? (Yes or No)
44 If so, were deductions made from (1) all, (2) one-half or more, (3) part but less than half, of wages or salary?
USUAL OCCUPATION, INDUSTRY, AND CLASS OF WORKER.
46 Usual Industry
47 Usual class of worker
FOR ALL WOMEN WHO HAVE BEEN MARRIED
48 Has this woman been married more than once (Yes or No)
49 Age at first marriage
50 Number of children ever born (Do not include stillbirths)
This information comes from pp. 62-66 in Measuring America: The Decennial Census From 1790-2000, part 2. At this source you can read the census taker's instructions and you'll learn that that there were 3 parts to the census. The other two included a census of occupied buildings and a census of non-occupied buildings.
Are you looking forward to finding someone's name in the 1940 census? I am!