U.S. Census records are kept confidential for 72 years as a matter of law. That is, the individual forms filled out for a census will not be available for public review until 72 years after that particular census.
Tomorrow the Census Bureau will release the records for the 1940 census, and if I read the website correctly, this will be the first time that the records will be accessible online outside of the genealogy websites that have been digitizing Census records.
If you are interested in finding people who would have been counted back in 1940, you will need to know some address information. There will be no name index available for searching, at least shortly after tomorrow’s release. Two genealogy websites have announced plans to index the records (I suspect there’s a story about the access rights granted to these sites for previous Census data that will ruffle some feathers), but until those can be accessed, you will need to know address information in order to find individual records.
Please keep in mind that the demand tomorrow could be considerable. Presumably the National Archives is mindful of this, but it may be a good idea to find non-peak times to access the website. Here’s a video the Archives put together for the census release. Happy researching.