The ACS is a nationwide survey designed to provide communities with reliable and timely social, economic, housing, and demographic data every year. A separate annual survey, called the Puerto Rico Community Survey (PRCS), collects similar data about the population and housing units in Puerto Rico. The Census Bureau uses data collected in the ACS and the PRCS to provide estimates on a broad range of population, housing unit, and household characteristics for states, counties, cities, school districts, congressional districts, census tracts, block groups, and many other geographic areas.
The ACS has an annual sample size of about 3.5 million addresses, with survey information collected nearly every day of the year. Data are pooled across a calendar year to produce estimates for that year. As a result, ACS estimates reflect data that have been collected over a period of time rather than for a single point in time as in the decennial census, which is conducted every 10 years and provides population counts as of April 1 of the census year. It is important to understand that the ACS was designed to provide estimates of the characteristics of the population, rather than actual counts of the total population.
Excerpt from: Understanding and Using American Community Survey Data
The American Community Survey provides vital information on yearly basis about U.S. demographic trends, jobs and occupations, educational attainment, veterans, homeownership, and many other topics. The table below provides a list of content collected through the ACS. To see what you can learn from ACS data, check out this interactive visualization.
Each year, the U.S. Census Bureau publishes ACS 1-year, 1-year supplemental, and 5-year estimates (note: 3-year estimates were discontinued). When choosing which dataset to use, you should consider: (1) the population size of your area, (2) currency of data, and (3) sample size/reliability/precision.
All estimates are published with an accompanying margin of error (MOE), which tells you how the estimate may vary from the true population value. The 1-year estimates provide the most current data but have larger margins of error than 5-year estimates. The 5-year estimates have larger samples and smaller margins of error than the 1-year estimates, but they are less current.
Use the U.S. Census Bureau comparison table below to help you select the right dataset.
For guidance on when to use 3-year estimates, see "When to use 1-year, 1-year supplemental, 3-year, and 5-year estimates" (U.S. Census Bureau)