From the webpage:
American FactFinder provides access to data about the United States, Puerto Rico and the Island Areas. The data in American FactFinder come from several censuses and surveys. For more information see Using FactFinder and What We Provide.
As I was writing this post I returned to CensusReporter (2013) which reported on an effort to make U.S. census data easier to use. Essentially a common toolkit.
At that time CensusReporter was in “beta” but has long passed that stage! Whether you will prefer American FactFinder or CensusReporter better will depend upon you and your requirements.
I can say that CensusReporter is working on A tool to aggregate American Community Survey data to non-census geographies. That could prove to be quite useful.
Enjoy!