In the latter part of 2012, for the first time, the U.S. Census made available a Web API (REST) for the general public to use and combine into "mashup" applications such as CensusConnect(api.census.gov). This API exposes "block level" census data which is often times down to the city block of the residence in question.
The census developer API is documented at the census website here: http://www.census.gov/developers/. This data is used to fuel the values for each of the census attributes displayed alongside the property listings using CensusConnect. Once CensusConnect is download, and the user displays there favorite supported real estate listings site, CensusConnect "connects" the address on the listing by making a census API call to api.census.gov, and retrieving all the block-level data for that property.
The derivation of the exact census level and census tables that are used throughout CensusConnect are documented here, but the lowest atomic unit for which U.S. Census data is gathered is the Block level. A census "Block" varies in geography depending on where you live in the United States, and the population density of your region. So, as the name "Block" would imply, often times this equates to a physical block, however in more rural areas, a Block can often times be made up of several physical blocks or even miles. As you can see from the screenshot below, the Block region that was surveyed contained 58 respondents, the BlockLevel contained 980, and the Tract contained 2962 respondents.
CensusConnect also provides comparative results at the BlockGroup and Tract level. These are also Census-defined geographic regions which are larger in area than the Block. Often times, a BlockGroup is made up of a dozen or so blocks. CensusConnect provides these results for these larger geographies so the user can have an idea of comparative socio-economics of the Block as compared to the larger surrounding areas.