Every query must include a geography, and this API supports both FIPS and GNIS geography codes.
To query the API your request URL query string must include a for argument which defines the geography level and FIPS code(s).
For example:
http://metadata.dsd.census.gov/data/2010/sf1?key=...&get=P0010001&for=state:06
will find the total population for the state of California.
The "for" argument may include additional FIPS codes, separated by a comma. For example:
http://metadata.dsd.census.gov/data/2010/sf1?key=...&get=P0010001&for=state:06,24
will find the total population for the states of California and Maryland.
In some cases you must also include an in argument to fully qualify the geography. For example:
http://metadata.dsd.census.gov/data/2010/sf1?key=...&get=P0010001,NAME&for=county:001& in=state:06
will find the total population for Alameda County in the state of California.
The APIs accept calls using a geographic FIPS code. These codes are available recursively through the API but if you have an application that produces a geographic coordinate point, here are two ways you can convert that to a FIPS code.