Census records vary by locale and year taken but generally speaking, they are lists of residents of a specific area in a specific year. In more recent years in the US, census records have included the name of every member of the household, their age, how they’re related to the head of household, and more.
US Census Records are a great place to find family members, figure out how they are related, and track their movements over a period of years.
Here are some quick links to US Census Record collections:
Massachusetts census records online–didn’t go thru individual county working listings
New Jersey census records online–didn’t go thru individual county working listings