DP02, DP03, DP04, or DP05. A full list of variables can be found here
B01001A_013E, DP02_0011PE, etc. A full list of variables can be found here
Single geography such as state:26, county:163&in=state:26, US or specific GEO_ID. A full list of areas can be found here
Your API key here
This connects to the US Census Bureau API, please review the Census documentation