U.S. Census of Population and Housing subject guide 

American Community Survey (ACS)

The ACS is planned to replace the decennial census long form, so that estimates of data on demographic, housing, social and economic characteristics will be available more frequently than once every ten years. When ACS is fully implemented, data will be provided annually for states, cities, counties, metropolitan areas, and population groups of 65,000 people or more. For smaller areas, it will take three to five years to accumulate sufficient sample to produce data for areas as small as census tracts. See ACS Data Dissemination for details and release dates, and How to Use the Data: Errata for notes on problems with the data and the status of their correction.

Where to get ACS data:

American Community Survey: Access Data (Census Bureau)

American Community Survey (ACS) Series at ICPSR

Estimates & Projections

Estimates and projections provide a limited amount of data for years between the decennial censuses.

The Census Bureau publishes total resident population estimates and demographic components of change (births, deaths, and migration) each year. The estimates are also published by age, sex, race, and Hispanic origin for the nation, states and counties. Estimates usually are for the present and the past, while projections are estimates of the population for future dates.

Where to get population estimates and projections data:

U.S. Census Bureau: Population Estimates
U.S. Census Bureau: Population Projections