My grand-aunt and grandmother, 1940s
Below are some helpful links to get interested researchers started:
Wikipedia: 1940 United States Census outlines what exactly you'll find out.
It's going to be a while before 1940 is fully indexed, so unless you know exactly where your relatives might have lived (perhaps the same house as in 1930?), you may need to do a "census crawl"—meaning you'd go page by page, looking for your relatives in the city or town you suspect they lived in—in order to find them. Or you can wait, like I intend to do. In any case, happy hunting!